I'll usually put a set of vice grips or a small pipe wrench on the round side and try removing the nut. Sometimes it works, sometimes it doesn't. If it doesn't come easy, I've resorted to other methods. I've used an angle grinder to put a slit in the nut and bolt (careful not to hit the springs) then taken a Craftsman flat screwdriver and beat the nut apart. I've also used a nut splitter to accomplish the same thing. And I've also taken a cold chisel and a BFH (Big Frickin Hammer) and whacked the nut off.
Do be careful, the spring will actually spring apart once that center bolt is removed. I've replaced them with grade 8 bolts before (you have to use a large C-clamp to pull the pack together again) or you can get a centering bolt from a spring shop. If you use a grade 8 you may have to either round the head of the bolt or slightly enlarge the hole on your spring pad on the axle.
While you got the pack apart, you might want to think of replacing the wear pads on the ends of the leafs. Any good spring shop should be able to provide replacements cheap. It'll keep the springs from squeaking.