Greetings!
I have been using UBB since back then and if you haven't resolved the issue let me know. I have a complete 5.45 version still in limited use that I can reference it if we need to walk through a few steps to remedy the problem but it is easy to remove a user with the classic version. /cgi-bin/Members - instead of root / or /ubb folder and a little poking around does the trick.
Just always back-up all files in a safe place just in case.
