I don't think that is possible without changing the whole thing so that when you want to log chats it sends an invitation of some kind to the contact for them to approve or decline (Good idea for a plug-in) But it wouldn't stop the person from logging you with another program so i think it's imposible, unless it becomes a new feature in MSN Messenger.
I don't actuly know much about this so i could be totally wrong so don't hang on my words lol.
/ 
Good idea for a plug-in) But it wouldn't stop the person from logging you with another program so i think it's imposible, unless it becomes a new feature in MSN Messenger.
so don't hang on my words lol.
