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 redistribution.
Keep in mind, especially in the states, there are extremely severe and exaggerated penalties for illegal filesharing and these companies have the means to do mass lawsuits. If you get caught up in one, you could be in a lot of trouble and it does happen.
For this to happen, all they need is an IP; if you're concerned about being caught, rent a VPN like BitGuard or SwissVPN. Then they can't pin it to you. Otherwise, however, all they need is an IP that is freely available to anyone else who might be a peer to the torrent (aka: seeding to you and leeching from you as you upload bits or download them).
Read in: Rent a VPN if you want to avoid getting caught. If you don't, you could end up being sued out of house and home, quite literally and the only info they need is easily accessible. If you don't want to risk it, don't download illegally.