To have uTorrent stop seeding when you reach a certain ratio, eg. 1.5, go to Preferences, Queueing.

Select "Seed While Ratio is:" <= 150% or seeding time is: (Ignore)

and then "When uTorrent Reaches the Seeding Goal, limit the upload rate to:" 0 (stop).

If you want to have certain torrents (eg. from public trackers) to stop seeding as soon as the download has finished, right click on the individuall torrent and choose Properties. Then under "Seed While" select Override default settings and select Ratio: 0 and time: (Ignore)