I needed to remove mine a couple of years back, and yes a chisel is the best way. I got hold of the smallest cold chisel I could find (about 4"/ 10cm long) and ground it to a point on one side - it looked a bit like a birds beak. That way I could get it in at the wider part of the bolt head and punch an indent in it before changing the angle of the chisel to to act around the edge and turn the bolt. Fortunately the bolts weren't rammed in with masses of torque so were fairly easy to start moving. Once loose I could easily remove them with fingers. when they were out I sawed a slot in the heads so a screwdriver could replace them (and remove them again in future if ever needed). Good luck.