If conflating the act of violently taking someone's life away irreversibly to displaying an offensive custom skin in a videogame sounds like a reasonable and genuine argument to you, I don't think there is anything anyone can say to convince you otherwise.
I agree with the parent comment that restricting customization for the offending users in the game is a reasonable punishment. They can still play the game AND have zero actual possibility of committing the same offense. The desired outcome of the user not using offensive imagery in custom skins is achieved.
Case closed, I don't think there is much left to say here then.
When your brother aims down the sights at you in Call of Duty and - gulp - pulls the trigger, you'll necessarily have him sent to jail.
That's okay, though - when you're counting on your fingers and get to three, and... hey.. are your thumb and index kinda.. touching? you'll be right there with him.
Also this was just an explanation of how it's an option, to the person that didn't understand how. You might want to do a full ban, you might not.
"Don't let them use customized cars" is extremely different from "ban them".