Marley,
In the summer 2001 issue of SeaScope, it recommends a mandarin. I am not sure if it works (I never had flatworms), but they eat live food, and constantly swim around the tank looking for food.
It also said that leopard wrasses will eat them, but should be dewormed with a product called Flagy® before being introduced into the tank.
Good luck in getting rid of them. If it works, let us know, it would help others in the future.