Soak the heck out of it with penetrating fluid. Make sure that there isn't a set screw holding it on at the keyway. There usually is. Most times it is an allen screw. In a pinch, use a deep socket or piece of pipe, around the shaft but not touching it, and lightly hammer untuil the rust between the shaft and the pulley has broken. BUT, double-check for that set screw first. When re-assembling, use Never Seize on the shaft and pulley, so this won't happen again. When all else fails, a little bit of heat, a little bit, will usually set it free. BUT, remember, you have gas and oil in the surrounding areas. Be careful.