two actually,
if you have a rotary tool (dremel will work best) take the thin cuttin wheel and cut a nice slice in the top of the head, therefore you'll be able to use a flat head screwdrive to get it out.
or
weld it. Take another piece of metal and weld it to the bolt (or screw or whatever), which will hopefully hold it in place so you can loosen it from the other side (or use it to turn it).
just a couple methods of how we solve some problems in my shop class
hope it helps.