I've been using a chain wrench for years(sold in plumbing supply stores or Sears) to hold the pulley fast while I remove the bolt.

 

Mine did the same thing, easy to change out, i did this in the parking lot at work in 25 minutes. I posted pics a longggg time ago, may still be up.

 

Just some info,(everyone most likely will already know this but) waterpump gaskets aren't the only thing that can leak on a waterpump, there is a seep hole that will start to leak when the waterpump is well...DONE. It can also leak out the from seal, check for play and noise.

Dustin

 

Air Hammer with a flat blade in it also works good for breaking the nut loose. Just leave the belt on to hold the pulley while you hit the nut with an air hammer a couple times. Put it back on the same way.

 

should be advised if you take it off with that method that you may not be able to use a wrench on it later if you had too