Normally with this type of anchor, if you back the original bolt out a little bit, you can then tap the head and either break the shouler off or straighten out the bend behind the drywall to allow removal. If the shoulder comes off, simply push the anchor into the hollow wall and then mud the hole. If the bottom poriton with the threads does not straighten out the material enough to remove the anchor, then as indicated earlier, a drill will usualy remove enough of the metal holding the shoulder to allow removal.
