I"m all about biological controls, and this one has never failed for me:
I just add a 6-line wrasse, and siphon out as many as possible before adding the wrasse. Within a week, if I have gotten all visible flatworms with the siphon, the 6-line has cleaned up the rest and they've never come back. I'll admit that I've only done this in 3 tanks, but it worked every time. And I love 6-line wrasses anyways, just needed an excuse to get some. Don't put more than one in a tank by the way, I have one in each of my 3 tanks. Psychedelic dragonets also keep flatworms at bay. I've never had flatworms exist in a tank with my psychedelic dragonet. I've had my dragonet for 4 years now!!!! In a 55 gallon tank with no visible pods! Don't know where she finds the buggers but she must.
Anyways, try a 6-line or 12-line/8-line as they are all very similar and reef safe.