in my board i can send someone to a site of my choice when they come on the board for a good joke.
However for this i use ban ip i have put an error_banip a refresh for a webpage (http://www.teletubbies.com :D ) but ip change so i want to use ban username for this
My php skill is poor so i don't see how do that
i think it's could be a good joke ;)

The best way is to do it by userid. So find out their userid and do something like this:
That needs to go somewhere in index.php.
You could put it under require('./global.php')