Not sure if you can do the old fake space or not for usernames (I won't say how but computer savvy people that know ascii characters know what I mean) used to create these in DOS for folder names and no one would ever know they were there (invisible folders), meaning they could put it at the end or beginning of any ones username and no one would ever know it was there and it would look exactly like the same username.
But as Worluk said when you ban a player in game it's not by username, sure that would be the way you'd find in game who you want to ban but it's by their unique identifier or number and can be done by IP, that they truly get banned by.