How can I make facebook not remember a username?

[Purple Penguin]

I was typing in my username and forgot to press tab before I typed in my password, then I pressed enter... Now whenever I got on facebook and click the box to type in my username, all the ones what have been used come up and the one with my password is there. Is there a way to remove it, or will it go away naturally (because some of my friends have used facebook on my computer and their usernames aren't there anymore)? Thanks.