First you need to loosen the top clamp bolts. You "can" use a large adjustable wrench, but risk damaging the caps (very soft and small shoulder). Like mxeagle, I made my own tool out or a piece of 1/4" aluminum plate. Simply marked on the 50mm octagon and cut it out. It's not pretty, but I was in a rush at the time and it works fine. I kept saying I was going to make a better one, but never got around to it.
The best and simplest way is to simply buy the right tool. It's available from Motion Pro, Race Tech and numerous other places.