@BoV, technically the userid must be in the database, I assume, since userid-s join the games, not usernames. So what could be done is to delete personal data connected to the userid, but you can't delete the userid for sure.
Also, if it were possible it would just open the way for trolling, multiing, and so on a lot of unwanted things.
I don't see the point of deletion since the only public info about you is your nickname, which in most cases tells nothing about the person itself. You don't get emails, nothing, why you would delete.
But to be completely honest, I don't know wheter it's technically possible here, I'm not a mod, neither an admin. I just don't see how it's possible fro a programming side.