In general, once a population of something like this reaches too large a number, you cannot biologically control them. Many fish like to pick at them, but none will eat an entire population.
They naturally regress and explode in waves and appear to generally vanish in most systems (although it takes a long time).
Flatworm Exit (product name) seems to do a great job of taking them out, but there are consequences (fish eat them thinking its food thats been added to the tank and die/harm natural good flatworms/etc).
If you do decide to use the Flatworm Exit, follow the directions carefully. As the flatworms die they release a potent toxin that has the potential to wipe out your tank. For all the ones you do see, there's just as many hidden in the rockwork. They must be syphoned out and water changes/carbon must be run after using the FE.
