The Harry Potter fanzone has a list of all of the spells that exist in the Harry Potter world. Here is one particular spell that I just became aware of:
Name: Homorphus Charm
Description: This charm will force a werewolf to return to its original human state.
As my friend Brett pointed out, this would have been really useful in The Prisoner of Azkaban and would have saved a lot of trouble.
The power of hindsight…