Newer versions of the popular BitTorrent client uTorrent feature sponsored torrents and a small ad unit on the bottom of the left-hand sidebar. If you'd prefer a less cluttered interface, you can easily disable these ads via the Preferences menu.
The settings you want are hidden away in the Advanced section of the Preferences window (see above). The two options you want to change are
offers.left_rail_offer_enabled (for the sidebar ad) and
offers.sponsor_torrent_offer_enabled (for the sponsored torrent). To quickly access them, just type "offers" into the Filter box on the Advanced window.
Double-click each option so it's set to "false", hit "Apply" and then "OK". You'll need to restart uTorrent, but once it's back up, the sponsored torrents will no longer appear and the left-hand ad unit will always show the "upgrade to uTorrent Plus" image. If that upgrade reminder is still too much, you can just hide the entire sidebar — which isn't that useful — by pressing F7.
Of course, leaving the ads on passively supports the developer, though you might already be doing that if you installed the uTorrent Toolbar that's now a part of the setup program.