If you realized that he was trolling, you should had acted immediately instead of acting like a sissy and asking him to stop.
You even provoked him by changing the welcome message.
And don't start talking about stuff like, "I wanted to give him a chance to stop".
This guy was trolling and obviously wanted the worst for your server. So why would you give him a chance? No logic in that.
Next time, just ban ban ban.
It isn't a loss to the server if one single person with a bad behavior is disbanded... I'm pretty sure it's leaning towards the other end.
Which gives me the conclusion. op is dumb