Stopping Output

I'm in the process of making a forum in PHP, and am wondering if there is a way to just stop sending stuff the the client(like if they're banned). I'm including a php file in all the .php files that make up the forum(reads the forum configuration file, sets up style sheets, etc). I'd really like to be able to put the code in there, so if/when I update it I won't have to update 20 different files. return 0; stops everything in the included file, but everything from file that inlcuded it still shows still goes on printing. Thanks in advance.