So it sounds as if the hex hole is rounded out. You could try an "american"-measurement hex wrench set, to see if there's one slightly bigger than the metric one, that'll fit in the enlarged hole. But I think this is unlikely anyway. If you can get anything to work with hex wrenches, it'll probably have to involve
(a) soaking the threads in question in WD-40 or liquid wrench
(b) very careful turning of the wrench while pressing the wrench head full-force into the bolt - think of a phillips screw that you're worried about stripping
If this doesn't work, best thing to do is use a power tool to drill a flat groove into the hex bolt head, so you can use a flathead screwdriver on it. This method is sometimes used with hex bolts on SPD-style cleats, where the surface has worn down and a hex wrench is no longer usable to loosen the cleats.