Sometimes a flat blade screwdriver will fit tight enough to turn one out.........on occasions I have took a small cut off disc and cut a horizontal slot in them and twisted them out....and remember, always try to "tighten" them first. Sometimes that will help brake them loose. Of course if you find a clutch head bit to fit an impact wrench as David said above, a good whack will do wonders....including twisting them off. If that happens, just drill them out.
If all else fails, grind the head off then twist them out.
