You always seed a little bit. That's part of the protocol and can't be helped.
Basically, just wait for your download to finish and remove it from the list of torrents. You'll have seeded some, but the people they usually catch are the ones that seed it for the purposes of enabling...
I could probably get it if I thought it out, but I don't even really remember the premise that well.
Anyhow, two requests:
1. Can somebody please give me a detailed version of it.
2. What is the solution supposed to be?
The riddle in question involves a Truth teller and a liar. Something to do...