This guide for Vista looks like it should still work:

 

http://www.winhelponline.com/articles/128/1/How-to-disable-Fast-User-Switching-in-Windows-Vista.html

 

The options are still there in Windows 7. (I haven't actually tried them, and I'm not on a domain, but they are still in the same place except that Local Computer Policy has been renamed to Computer Configuration in the tree on the left of gpedit.msc.)