Natural predation may help but is somewhat unpredictable. Dragonets and Leopard wrasses have been used with varying degrees of success. Chelidonura species of sea slugs also known as the Greek Goddes - have also been cited as excellent natural predators, but acquiring an effective species is difficult for many livestock resellers, as collectors often lump various nudibranchs into an assorted category. The result is that numerous species of sea slugs unsuitable for captivity get imported in an effort to acquire just one that eats your species of flatworm.
In the meantime you can try to manually siphon them, I've done it but it is a PITA. You can sit and watch because many times you will find that they simply drop in population by themselfs (I'll be looking forward to Waterfaller telling me that she has found some more, hope not but expecting it). (someday).
They also tend to hate heavy water flow and can then be sucked up by your canister, overflow or other mechanical means. Heavy skimming is said to also be albe to help control them.
Asbury - If you have flatworms can you take a pic or describe them? color, dots, stripes?
Thomas