Hello, they make a special tool that holds the pulley while you turn the big nut to remove the clutch. You can rent the tool or you can try backing 2 of the bolts out of the pulley and use a flat pry bar between the bolts to lock the pulley and keep it from turning. Some guys have good luck with just taking a large punch and hitting the nut on the top left side with a good strong sharp blow from a hammer. I prefer to use the special tools. Picture below. It is a regular thread, left is loose. Thanks
Auto Tech
Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)