I hate to tell you this . . . but welcome to the polyclad club. Its the club you don't want to be in.
I have one in my tank that I am unsure of how to get rid of. BUT I can tell you what NOT to do . . .
1. Do not try grabbing it by any means (ie tweezers, etc) . . . it will tear and now you have two.
2. Don't buy snails until you get rid of it. This will merely feed and cause it to reproduce. An expensive snack.
3. Syphoning it wont work either as it will just tear and like grabbing it, now you have two.
Let me know if you think of something . . . I havent yet. I tried a baiting tech, but did not work as planned. the thing came and went before I could catch it. I may try this again with a slightly different approach.