There is a tool available that will hold the pulley in place, check out eBay. It does take a bit of effort to undo from my experience but you do need to get a good purchase on the "nut" as a slip can cause damage to the nut and or the tool itself. I believe it is recommended that you replace the "nuts" once they have been removed, I did when I last did this job but i did have a spare supply of "nuts" and I wasn't going to take any chances.
