Replacing a fan clutch.

They actually make a special tool for the process. Most parts stores will lend them to you. However, I never had much luck with them. What I do is this. I have a large cresent wrench that I place over the retaining nut. Make sure to leave the serp belt on to help prevent the water pump from turning. Then I wedge a screwdriver between the pulley and pump to help keep it from turning. With a dead blow hammar, smack the cresent wrench with the hammar. It usually takes about 3 or 4 good hits before it breaks loose.

Let me know if this helps.
Joe