One way of doing it, albeit not with .htaccess is to have a standard login form, with username and password, and put the usernames in a dynamic pull down from a db.
When they select their username, the password doesn't automatically appear, the save password feature isn't available.
Note
I don't know if Gator would override this...
As Vincent said though, can't do it with .htaccess...
Why are you trying to do it though, I may have a better suggestion if I knew why you were trying to disable that feature.