all of the above will work. many people also have success with kalk paste (also mentioned above, but that was jsut an example), lemon juice, or boiling water.
i had some waaay down deep in pockets so as soon as i would get near it, it would retract too deeply for me to reach it, so i either broke off the tiny chip or rock it was on and let it dry out and die, or i filled the hole in with some hold-fast epoxy