The title pretty much says it all. Here is the link: http://www.dynamicdrive.com/dynamicindex9/noselect.htm I want to use it to stop plagiarism. I know it wont 100% stop the problem altogether (because of "view source") but I believe it is still worth having.
Spiders cannot run javascript so it will not impact them but as you say, it wont do what you want but will annoy a lot of genuine visitors.