I don't know about you people out there, but I'm awful sick of the del.icio.us Firefox plugin adding itself to my list of search plugins. At the top no less! Well I've beaten it! These instructions are OS X specific, but it would't be hard to do a similar thing in linux / windows (you'd use chmod on linux, and the ACLs in windows).
Simple really - after removing the search plugin from your list (with the SearchPluginHacks extenstion) open up a terminal, and type the following: