Hi,
I would let them be...flatworms are not really a big problem....until they completely cover your corals. They are just not really pretty -- maybe if they were multi-colour....
But, the best way is to siphon with a canister filter
That way, you can spend more time sucking up flatworms without draining all of your water. It doesn't need to be a canister filter -- any filter will work.
Chemical control of flatworms is really dangerous -- it is known to be very bad for SPS systems. Biological control...it is not a good idea if you have not prepared the system for these animals. Popular fish that "may" eat "certain" types of flatworms include Mandarin and 6-line wrasse. It is a really hit and miss....more misses than hits. The other problem is that once the flatworms are gone...can you keep a pod-eater happy in a 15 ? Maybe, if you have a refugium of some sort.
Hope that helps.
- Victor.