Imagine we suddenly found that an undersea volcano was erupting right into the middle of the Gulf Stream. If allowed to continue it will cause severe disruption to northern Europe, particularly Ireland, Britain, and Scandinavia. So how do you prevent it?
ROBOTS!!!
Gigantic robots with LASER BEAMS shooting out of their eyes!!!1111!!
That always works!
<cue crickets>
OK, ok, ok… I got nothing.
You can’t.
Not likely of preventing any bit of it. Imagine the Hawaiian Island Chain starting up in the gulf stream. It took at least 50 million years to get to the size of those islands, so I believe humans can adapt over time w.r.t. the changes in climate. In that link there is an interesting effect the islands have on the Trade Winds that disrupts the flow of air over a sizable area in the Pacific and may explain in part the El Nino effect. I can imagine something similar happening in the Atlantic depending on how the islands are oriented.
We are lucky to have a small oil volcano in the Gulf of Mexico stopped for the moment and the future of that isn’t even certain. That is just a pipe that people put there in the first place. Humans would be powerless to stop a real undersea volcano in terms of technology and organizational (government) skills. The best hope would be to detonate huge nukes on top of the volcano but that probably wouldn’t work either.
Shit happens.
If it’s not enough to put an entire ocean on top of it, what else do you think humans could do?
Rubbish, robots are too weak!
I’d hit it with a chunk of antimatter the size and density of Ceres.
No planet, no volcano.
It’s a slam dunk!
I was going to suggest Imodium but I just realized I misread the title as “erupting underwear volcano”.
But we wouldn’t need to plug the volcano, merely 
stop it affecting the Gulf Stream. So could we periodically collapse it, for instance? Say by drilling into the side and detonating suitable explosives?
Wouldn’t that just make it bigger? The reason it is erupting is the enormous pressure of the magma has broken through the earth’s crust; detonating explosives would only make the opening in the crust bigger. Realistically, explosives would probably have no effect whatsoever, as the amount necessary to affect a volcano would be astronomical,
Nuke it from somewhere.
Well, then it sucks to be Northern Europe. Anything that is attempted would be rather feeble compared to the power of even a small volcano. Nuking it would probably open up the lava flow more.
We’d get more cakes and winking soccer players for Portugal.
This brings up the question of why humans keep thinking that they can control natural events at all. We are a pretty smart species but still little piss-ants in the overall scheme of things. My home state of Louisiana got hit twice in the last five years with horrible disasters. The first was Katrina which should be an easy one if we had as much power as we think we do but that was a mega-fail. Then, there was the Gulf Oil volcano which was an epic fail because it was engineered by people and still couldn’t be stopped. We will have to see if it is fixed but there is at least a generation’s worth of damage already done.
Science fiction writers everywhere make me sick. It is one thing to dream about conveniences but we are just smart apes and we can’t control our own planet let alone blast out into the interstellar world. The federal government probably isn’t as competent and efficient as the one you work for (think about that for a minute). It is basically proven that they couldn’t handle a dirty nuke going off in Manhattan or a giant earthquake in California or even Missouri. Thank God those haven’t happened yet but something like it will.
None of that compares to an undersea volcano though. That is hopeless.
But where? Hmm, if only we could think of where to nuke it from. Not the Earths surface, surely, but where, where? Where to nuke it from.
Actually, I recommend putting out a distress call to whatever galaxy class starship is in our quadrant and have them reverse the neutritium flux of our sub qualactan particles. That should calm the crust down.
It’s never failed.
Ok it’s never been tried either, but it’s never failed.
We could put solar sails on a large asteroid or comet, directing it to Earth and if we can put it into the correct re-entry path, it will land on the volcano, demolishing it.
Wha? But I’ve seen the historical documents.
It’s the only way to be pretty confident.