Help with difficult player

Ok. I'm new at this and I don't expect mercy. I just want some help.
I have a bunch of friend on discord with who i do some roleplay, often homebrew rules based on savage world or anima or dark heresy.
Here is a thing. Among them, I have a Mary Sue with some tendency of that guy. We keep him because... He does have some redeeming quality. When he is not fawning over his DMPC, he is actually a good DM and he often let his DMPC offscreen or god know where while we do things.

But his mary sue characters royally piss me off.
No wonder in my star wars campaign, he is the only one with missing members (right arm, artificial lung). How he got in this state ? " I charge the two inquisitor !"
" what ?"
" I'm force sensitive and i have a light saber ! Not mention i descend from a long line of Grey Jedi."
" yeah may be but... I have your character sheet under mybeye. You are not trained for this. Either force fighting or laser sword style."
" not even one ? "
" dude you discovered your power one week ago."
" Well Rey did..."
At this point, Inquisitor A cut his hand and the other was preparing him to be detained.
The others players finally rescued him... By using the weapon of their ship. That is how his lung got pierced.

In another campaign, warhammer 40k, i told them they were free for their character. I even tolerated some xenos.
Boy. I never thoses words ever again. He brought me his own craftworld of mary sue.
I mean. I had another player who played the lord of a planet... Who was being blackmailed by the inquisition because an inquisitor had proof of a genestealer cult on his planet and that he was most likely infected. Just not under control yet.
Him ? " I'm a psyker, wraithbone singer !"

Maybe i'm too tolerant. But at that time i didn't know a lot about eldar fluff.
When he tried to sing a shuriken gun, I just asked him if he knew how it worked. He still tried to negotiate.
At the end... Well i would like to say i killed him in a imaginative way. But no.

He got finally killed by his teammate when he said he was tired of warhammer 40k.

The thing is, I will play a new campaign. I actually let them vote the universe each time. I do some research and i cook something ( generally just the beginning and the end because i have a grendel player. As much lucky).
So what would be your advice to deal with him without hurting his pride ? He is still one of my friends.

I don't understand half of what you're saying. Concisely explain what's the problem independent of examples.

But already have a preliminary

>have you tried talking to him?

Sorry. French dude here, and not as skilled to write it as to read it.

Yes we actually did. And in " we" i include me and the 5 others regulars.
Basically I have a friend who often play mary sue/min-maxed character, turned up to 11. I want to deal with it to not bother my others regulars.

I see that you want to keep stuff as open as possible for them, you might have to cut that down a bit for him. Give him parameters, say: this is the powerlevel we are going for, make a character in this range. If he doesn't, tell him it doesn't work as if it's a violation of the rules: He has not made a character compatible with the game.

If he just comes up with shit like "I'm a grey Jedi descendant so I can do this" either just tell him no or again, give him clear parameters that aren't framed as ARGUMENTS but as RULES. The game has rules, there is nothing to be said about them.

The basic gist is: Use rule 0. What the GM says goes. When it comes to power restrictions, don't explain, present it as inherent to the conceit of the campaign. Leave no area for argument within the campaign - Instead, if he doesn't like it, he needs to talk to you about THE KIND of campaign he wants to play, and if he gives you a setting and power level, maybe you'll have a campaign or two where everybody is a mary sue and it's all good fun.

Thank you sir. I will try that.

I don't even read what to english. How is?

Alternatively, set very strict parameters for the the entire party, to the point of making all their characters and letting them pick which to play at random, and only give them leeway to fill in minor details or make limited choices.

"You're all playing human fighters. I've rolled up your stats. Pick a sheet at random, name your character, and pick 3 feats."

Tu peux essayer de m'expliquer dans la belle langue du saucisson si tu veux.

J'ai un pote qui joue quasi uniquement des mary sues et c'est gavant. Il essaie constamment d'obtenir des avantages via le background de son personnage et c'est lourd. Si c'est une simple connection dans un village je veux bien. Mais quand le mec te ramene un craftworld eldar qu'il a créé avec des regles speciales...