Google doesn't offer a way to turn off SearchWiki other than logging out from your Google account, but you can install a Greasemonkey script that hides the interface for this additional functionality. If you use Firefox, install Greasemonkey extension, restart the browser and the add the No SearchWiki script. Google's search results will look clean again, even if you are logged in.
The script also works in Opera, which has built-in support for userscripts. Just go to Tools>Preferences>Advanced>Content>JavaScript Options, choose a folder in the "User JavaScript files" box and then save the script in that folder.
Note that the script doesn't disable the SearchWiki interface for the results you've already promoted so you can restore their default position and you can still see the removed results at the bottom of the page. To undo your changes, you can use My SearchWiki Notes page.
Update: You can now disable SearchWiki from Google's preferences page.
