You can stop seeding after the download has completed by right clicking on the listing in the uTorrent/BitTorrent client and selecting 'Stop'. If you don't use uTorrent for anything else, I would just close of out uTorrent as there's no need to open it up again.
If uTorrent keeps launching for you on startup of Windows and you would like to keep it on your PC without it starting up with every Windows startup, hit 'Ctrl + P' for Preferences and then in the General tab there should be an option to uncheck "Start uTorrent when Windows starts"
If you'd like to seed the torrent again (completely your choice), press 'Start' and you will begin seeding it for others.