I have no idea about the answer to your question, but in case you’re interested in fiction about it, the Charles Stross novel Iron Sunrise describes the aftermath of such an event in one of the opening chapters.
A couple of really interesting answers to this question here
What would happen to a star if a Dyson sphere lined with mirrors reflected a significant portion of the stars light back to the star?
Unfortunately these answers don’t agree with one another. very well. They do suggest that the rate of fusion would increase, but the star would also expand and cool down more quickly, so much of the effect would be dissipated. An expanded star with a slightly lower density would be marginally easier to starlift.
If 10% of the Sun’s luminosity could be dedicated to lifting matter off its surface, it would be all gone in less than 100 million years; rather the Sun would shrink down to a brown dwarf, and fusion would stop some time after that.
Spin the star faster and have it fly apart? Since the star is a great power source it might be possible to use it’s own energy to do this.
Antimatter somehow released in the core with enough energy to blow it apart, or just throwing a stellar sized antimatter star at it?
OP asked for “a way to artificially stop fusion from occurring in a star.” If the star is blown apart, nothing happens within it because there’s no star left. Thus the goal is to quench the rather energetic fusion process and leave behind a sub-stellar mass.
The solution is simple: Squirt a shitload of oxygen into the solar core where it combines with the hydrogen, leaving a big blob of water tinged with helium. But let’s do that in another solar system. Ours still has its uses.
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we don’t have that much oxygen
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based on this column by Cecil: How many ice cubes would it take to put out the sun?, it still wouldn’t work.
Not a bad idea. Of course, spinning a star to raise material off the star into orbit would take just as much energy as any other method of starlifting, so this method doesn’t seem to have any advantage over other methods at first sight; but if you spin a star rapidly enough, it starts to emit energy in a band around its equator, becoming a so-called Be star.
This added luminosity could speed the extraction process up slightly, but I’m not sure it would be worth doing. At the end of the day you are wasting energy by spinning material you don’t need to extract.
I should have specified that I was using the Isaac Arthur definition of “possible”. 
Star lifting is a good answer, but I was more interested in stopping fusion without removing mass. It looks like the answer is “it’s impossible.”
Of course, if protons don’t decay, you could wait around until the star turns to iron, but that’s not too artificial.
In the Schlock Mercenary universe a race of sophonts use meta-stable dark matter to compress the core of the star into an oxy-neon-magnesium white dwarf, with attendant supernova boom - my understanding is that this would basically halt fusion (or significantly slow it down) and produce continuing energy primarily by thermal radiation.
The flaw in their plan is that the meta-stable dark matter gives rise to a race of dark-matter sophonts who primarily use gravity as a weapon and hate baryonic life with a passion …
Best start from the beginning if you are stuck at home in lockdown and need diversion for several days - it’s a 20 year archive of daily webcomics without any missed days.
Give the star a lifetime achievement Academy Award. That pretty much means they’re done.
But for how long? A star attracted by the gravitation of a black hole from a substantial distance would typically reach an appreciable fraction of the speed of light by the time it approached the event horizon. If the black hole was not particularly large, the star would be shredded by tidal forces and perhaps form a spectacular accretion disk for a while. If the black hole was huge, the star might just quietly disappear, and “freeze” at the event horizon where from our perspective there is no time, but it would emit no more, neither radiation nor particles. It’s not exactly “halting fusion”, but it’s a convenient way to make a star disappear!
There isn’t enough pressure in the cores of main-sequence stars to actually squeeze the nuclei of atoms close enough together to fuse–the fusion happens through the occasional bit of quantum tunneling, which is based on Heisenberg’s Uncertainty Principle. So if you surround a star with Heisenberg Compensators set to maximum range, maybe they could switch it off.
We’ve gone off the ledge here. I down to suggesting teleporting Tribbles to the center.
Take control of the Sun’s magnetic field. The Sun is already depleting itself through coronal mass ejections. The Sun’s magnetic field is dynamic. Fluctuations in the pattern of the magnetic field create a phenomenon known as magnetic reconnections which result in the “release [of] large quantities of matter and electromagnetic radiation into space”. According to Wikipedia, “the average mass ejected is 1.6×10^12 kg.” The Sun’s mass is 1.989 × 10^30 kg. Using the figure from post #15 that the minimum mass required for stellar fusion is 1/12.5 of the Sun’s current mass, you’d need a bit more than 1.6 trillion coronal mass ejections to reduce the Sun’s mass beyond that point. That calculation presumes that the average size of the coronal mass ejections will remain constant as the Sun’s mass diminishes, which probably isn’t true, but will give you a starting point.
Of course, that method is stopping fusion by the depletion of the Sun. If you want it to remain intact, you’ll need to find a way of creating an equilibrium between the Sun’s gravity and the electric repulsion of the protons in its atoms. That’s probably a bit more difficult.
Note that I probably should have read the entry on star lifting before I posted the previous answer. It looks like it encompasses the method I’ve just described in the first paragraph above.
Does dark matter count within “known science”? Lots of unknowns about it, but we know it can pass through visible matter and has gravity. So that gives more scope for potentially making a star expand (if you can throw around vast amounts of the stuff very precisely), which will temporarily pause at least some of the fusion. A whimper-nova, if you will.
Although, probably other processes would be releasing vast amounts of energy during such a turbulent process. I’ll leave it as an exercise for the reader to do the maths
Dark matter has been suggested upthread (read Schlock Mercenary for one strategy concerning the use of this material). I think the consequences of adding extra dark matter would be to increase the density of the star, which would increase the rate of fusion, not decrease it.
Oh I missed that. But anyway my plan is not just to “add” dark matter. Since dark matter can move freely through ordinary matter (and, apparently, itself) you have several degrees of freedom in the position and velocity of dark matter relative to the star. That’s the whole point of using it.
There should be configurations of dark matter where you can, temporarily at least, pull the star apart. With less pressure on the core, fusion goes down. But, like I said, there’s no way you can be throwing around hellatons of mass around every second and not release crazy amounts of energy for other reasons.
I’m not familiar with the metric prefix “hella”. Is that a multiplier of ten to the bunches?
10^27
Although on checking it’s not 100% official yet as so far only Google and Wolfram Alpha have supported it (link).
If you could coax a black hole into close orbit perhaps?