[kh99]

If you create a plugin using hook newpost_process you can check the message text and add to the $errors array, and it will do what you describe. The code is in includes/functions_newpost.php, and the post text is in $post['message']; so something like:

if (some test on $post['message'] fails)
    $errors[] = "Some test failed.";

I can't remember if you need to call return after that or not. Try it without and see what happens.