For years, I've suffered the frustration that I would type the wrong user name into some web site, and after that, the wrong name would be a permanent suggestion in the form history. It would confirm me if I started to type the same wrong user name again, and if I knew that I didn't remember the right user name, it would give me wrong alternatives.
I finally got annoyed enough to look for a solution and found it here. It's easy, and I'll bet it works in other web browsers too.
Basically, you just highlight the wrong entry and press shift-delete.