This would be an extremely time consuming and potentially destructive process.

First, take a grinder and chop off the tips. Then go to the hardware store and get a 1/8, or maybe 3/16, bit that you think is long enough (note- these are expensive). Then go home. Tap a pilot divot in the flattened tip of the lag bolt to get started. Drill your bit all the way through the bolt without snapping it. Take frequent breaks to cool the bit and avoid dulling the edge. Note that bolts tend to be made of quite hard steel to deal with the high forces they are put under, so expect this to take an annoyingly long time. Then do it again with a slightly larger drill bit. Then again. And then again. Etc. Until your bit is just barely smaller than the bolt. Finally, you go in with a drill bit as large as the bolt, and absolutely destroy all your hard work, tearing down the sides of the bolt in a haphazard way. With a large amount of finagling and cursing, you manage to set the bit in thr hole in the top of yhe stripper pole, and finally completely destroy the bolt, so the remains fall on the floor below.

Far more expedient would be using the grinder to grind the tip off, then cut a slot in the flat surface you made. Or maybe a +. Then put either a flat or phillips bit in your impact driver, and reverse the bolt out. How well this works depends on how hard they cranked down the bolts. If this fails, I would get a hole saw, cut around the bolts, and clamp some vice grips to them to turn them out from above.

But really, I would call a real estate lawyer and force my neighbor to pay a contractor to do all the work for me.