There is actually a tool that has jaws that grab the cap on two sides, somewhat like an oversize pair of pliers. Most folks don't have these, and a large pair of Channel-lock pliers will do the trick. The tool will leave a mark, so if the cap is exposed, you will instead need to get a wood chisel or similar tool and tap it in behind the cap a little at a time, working your way around until it's loose. It will come off one way or another.
