trust her or block msn all together, p.s. even if you do block her adding contacts on your home computer, she can just add contacts using another computer or add them using the web based messenger, you will be starting a fight which you wont win basically 
wormvortex said:you should try telling your daughter why not to add random people and learn to trust her. Restricting a childs internet access will just cause them to respond with trying to get round it or loosing faith in their parent.