Will, as Gryph said simply logout, and change your password. Why am I repeating this? Because I also have an explanation as to why he can't get back on:
Internet explorer will, if you have the option turned on, save information typed into input fields. This, however doesn't work on fields marked as password fields.
And by making the forums a favourite won't change a thing either, favourites have no affect what so ever on cookies. Cookies are what control your log in, unless you use the slower (and much worse) method of session hashing.
Either way, if you change your password frequently and logout then they haven't got anyway to get in. Oh, and make sure your friend doesn't see you type in or tell him it. Well, actually they could... But that's a bug that only I know on here
If the above don't work, simply threaten the guy, then punish him.
Alcar...