Since the carriage bolt heads are apparently above the surface given the
comment about the screwdriver, the easiest thing to do is to use a
hacksaw (or angle grinder with cutoff wheel) and make a slot in the bolt
head for the screwdriver.

Did that just yesterday while dismantling an old tent trailer. I use a
GIANT Dremel tool (Rotozip) with a 1 1/4" cutoff wheel to cut the slot