There is a special pulley holder tool that you can purchase. Snap-on makes a real good one. But there are several different ways to loosen them up without the tool. Leave the belt on so there is more tension. You can either hold the pulley with same channel locks or you can lock the pulley with a flathead screwdriver or a small pry bar by placing the screwdriver in between two bolts and holding the driver so that when you loosen a bolt the pulley doesn't jump. Or typically if you have a good air ratchet, it'll usually just loosen the bolts right up.