Well, If you have true planeria they are non paracitic. They feed off of decaying meat, so, they really just make your tank cleaner. They are pretty cool little guys, IMO. You can trane them to respond to stimulus. They can also teach others the same responce. Sorry, I am a zoology nerd...
If you want to get rid of them they are very vulnerable to extreams of temp, lite and pH.
So, you could leave the lites on for a while, and most of them will retreat into the shells and yo could remove them by hand, but I doubt that you would get them all this way.
A more effective way would be to take the fish out of the tank, and turn the heat up to about 90 Deg. And leave the lites On.
BTW, don't stop feeding them, this could initiate mating...