A while back, I was playing with Joomla, and I created a joomla user. That was fine, but then I noticed that GDM (the login screen) was offering me the chance to select that user to log in. That was not my intent.
A quick search revealed this useful post: http://ubuntuforums.org/showthread.php?t=1321845
GDM will show you every user with a user ID of 1000 or higher. I changed my joomla user’s user ID to 987, and voilà - no more prompt.
Luckily, the user I modified didn’t own much - and everything it did own was in a group that also owned the files. If that hadn’t been the case, I would have had to change ownership of the files as well.