Are the pads themselves turning when you try and turn the nut? If so, you need to hold the pad with a pair of pliers or get someone else to hold it why you turn the nut. If nothing is budging, you need a longer wrench or put a piece of pipe on the wrench handle to give yourself some leverage. And, if all else fails, you can cut the pad off at the threads with a hacksaw (been there, done that!).

Neal