yes, I have, Salifert Flatworm Stop will take care of them. Syphon as many out as you can out before using the medication. A piece of airline tubing should work pretty well. Its a PITA. When the Planaria die they release a toxin or some crud that can potentially wipe out the tank, so it is recommended to do a big water change right after treatment.
There are natural predators that eat them, head shield slugs I believe, but the slug starves to death after population has been decimated and a few straggler flat worms will be enough to repopulate. There may be some fish that eat them as well
I could be wrong , the blue hippo is probably not related to the fw's