It's not quite that easy.
You have to delete each of the stupidly-highly-numbered users, then delete the last_number.cgi file, then run a member rebuild through your control panel.
If you don't delete the stupidly-highly-numbered users, then the board will automatically "correct" the last_number file the next time it can.
(This being said, there's no way that a login name using all numbers would cause the problem you described.)