Although Maple syrup can be made from many other maples. Back in the days of mailing lists, someone i shared a mailing list with said that he won local competitions for the best maple syrup using partially sugar maple sap, partially (i think) silver maple sap, and his secret ingredient, a dollop of sap from moose maple. (Also known as striped maple, and many other names.) The real secret must have been finding a moose maple large enough to tap, as they are usually small, understory trees.
Not sure you said, yet are you willing and able to do this with a pick & shovel? That 18” stuff will have to be sunk about 21” to allow grass to grow above it.
My mom lived in an HOA and her neighbor was the head of the committee. The rule against planting trees on the border of your property (perhaps planting one at all) therefore didn’t count. Years ago I’d so some gardening alongside her house and every year there would be feelers and sometimes more root stuff to tear up as they got in the way and robbed nutrients.
Ten years later when I was there for a couple weeks - it was hopeless. The tree was clearly testing if had gone far enough (a yard or so beyond it’s now much bigger bough) or should it test the foundation of my mom’s house.
To handle this would take a backhoe and no way will this neighbor help with the bill.
Another factor is that damage to that tree’s invading roots, if severe enough, could fatally injure the tree.
While local laws might absolve the OP given the nuisance on his property, an aggravated/aggravating neighbor might decide to be a PITA and make legal trouble anyway.
So caution is advisable before taking any drastic steps.
Kill the neighbor first. It’s the only way to be sure killing the tree doesn’t cause trouble.
Crazy idea maybe, but how about tree root cleaner that is dumped down sewer lines that have problems with tree roots? They claim it will kill the tree roots but not the whole tree.
Trench several inches down along the fence, doesn’t need to be very wide and sprinkle the granules and then cover with dirt. Might stop the runners when it reaches this area and keep them from expanding into your yard.