I just glanced at mine, it's a plastic arm with a metal insert molded in, so this will be slightly tricky. Warming it will help, but stick with a hair dryer so you don't melt the plastic. A penetrating oil will help, I really like PB Blaster, but it has some strong solvents in it that might damage plastic. Try WD-40, and let it sit on there a day. You can't use a pulley puller on plastic, unless you can get a piece of metal under the arm to pull against. Tapping on the post might help loosen the arm, but put the nut on the end of the post to protect the threads when you do it.