I'm assuming that you've got an '86-on car, right?

I'm sorry to hear that you've already wrenched the nuts -- that must have torn the studs loose in the plastic housing to spin freely. But maybe you haven't done this on the other side yet. If so, you may be in luck. When I had to do that (to replace them with Euroheadlights) on two of my cars, I sprayed them ahead of time with PBBlaster to loosen the corrosion -- a few times a day, over two days, I gave the nuts a squirt. Finally it was time to remove them, and the nuts turned freely, with the housings coming out with their studs intact -- this worked on all the studs on the headlights on two cars (both '93s), so I recommend it as a good plan.

For the ones that are already spinning, I think the only recourse is to use a dremel with a small cutting disk, to cut the ends off. Sorry.