Player abusing narrative freedom

I’m running a play-by-post homebrew game. The game is narrative-driven, so naturally I allow players to make up their own races, backstories, abilities etc., as long as they “don’t overdo it” (as I put it).

The problem is one player who doesn’t seem to get what “overdoing it” means. At first he wanted to make his player an all-out superhero, and I had to talk him down from it. This was no easy task. This guy is well-meaning, but he doesn’t seem to realize that this isn’t a single-player power fantasy.

I tried explaining to him that he would outshine the other players, and completely derail my plots, not to mention that the theme of my world is being tossed out the window.

My reasoning seemed to help at first, but now he want his character to be a god.
Literally. In a gritty, post-apocalyptic cyberpunk setting.

Part of the problem is that he’s not even asking; he’s building his own world, and just keeping me updated.

This puts me in an uncomfortable situation where I have to either bluntly veto his posts (and there sure are a lot of those…) or start negotiating with him.

He really means well, and I don’t want to hurt his feelings, but I can’t let him keep this up.

So… any elegant way to put him in place?


Update:

I’m the sole GM.
There are 5 other players.
We are not part of another community.
It’s a forum, with major updates about once every two weeks with smaller updates in between.
I’m basically in command, but I announced from the start that players have freedom even to determine minor outcomes of their actions.