In your BOINC manager (make sure you're in advance view), on the "Projects" tab, highlight SETI@Home and click the button "No New Tasks". Then, on the "Tasks" tab, highlight all the SETI@Home Work units and click "Abort". It will ask if you're sure, and you are. That will fully rid you of the SETI WUs and prevent you from getting more.
And SETI will always show up in your stats pages. You want credit for your work! ;-)