What size tank and how long has it been up? What other inhabitants do you have?
In my mostly SPS 120, I first tried manual removal. Sure I got some, but I like to think I have a life beyond flatworms. I then tried a green coris wrasse. It ate some, but then went rogue and started eating shrimp. Needed a #10 fly hook and some line to get him out. I then tried Flatworm Exit. It certainly knocked them back, so after a week, I dosed it again at about double strength to no real effect. They came back in force.
Then I got a pair of Blue Star Leopard Wrasses. After an abbreviated QT, they went into the DT and started eating the little buggers on their way in. They didn't stop except to sleep until they were gone. They were even able to pluck them off my Elegance Coral without disturbing the coral at all.
Certainly the most effective FW eaters I am aware of. As a lucky bonus, they are not only model citizen, they are probably the most visually stunning fish I have ever owned.
hth