Flatworm exit works.
http://progressivereef.com/proddetail.php?prod=flatworm
But take some additional precautions when using it like doing even larger water changes to be safe. Also, you may need to treat a few times and then dip everything new that you buy so that you dont get them back.
Some wrasse are reported to eat them as well. But those are hit and miss. I've tried two six line and a yellow coris wrasse with no luck.
The Blue Velvet Nudibranch will also eat them but it wont be able to find them all and will begin to starve to death and leave a few behind which will eventually repopulate your tank.
They really dont seem to like a lot of flow so adding some more powerheads etc should lessen the amount that you see.
You can also manually remove them by siphoning them off out of the tank when you do water changes. But again this will only lessen the amount that you see and wont get rid of them fully.