If you are having issues with students abusing the "Send message" feature of Mahara, then you need to look for a line in user/view.php that looks something like:

$smarty->assign('canmessage', can_send_message($loggedinid, $userid));

(line ~298 in the 1.2 series) and change it to:

$smarty->assign('canmessage', 0);

That will prevent anyone from sending messages.

This really needs to be a configuration option. I might have some cycles to work on it and get it into the main branch.