to remove spots from iron and silver based process you need to bleach the spot away.
For silver processes
(it doesn't really remove the silver it just changes the colour to a gold / yellow)
For iron processes
household bleach: White King etc
(again it only change the mark to a golden yellow but if the spot is light to start with it may disappear)
the best way to remove a silver and iron based spots is to cover the spot when you are exposing the print. When it is put in the developer it should wash away but if it does show up it should be lighter and the bleaching is more affective.