happened to my rear caliper once. so i drenched it with PB Blaster and let it sit over night. then i locked some vise grips on the pin so the whole thing looked like ^. then i set it on a piece of wood and started pounding on the highest point with a 12 pound sledge hammer. after about 10 hits it started moving. then it just took alot of twisting to get it out. after i got it out, i used a dremel with a wire brush cut down in diameter and more PB Blaster to remove all the rust from the inside of the guide pin sleeve. put a very generous coating of grease, then put it all back together.