Helloooo karineee, just desintall msn and i think it will be ok! but i'm not sure!

I have found that using the "Real" MSN Messanger program to setup the contact settings helps this problem. There you can say only allow people that I have added to send messages.

It would be nice if Adium added these options as well as I was drowning in a sea of pointless pussy messages!

You can do it with Adium as well. Don't go into Adium Preferences, instead use to top menu where it says "Adium" next to the apple logo (leftmost side) underneath Preferences you'll see "Privacy Settings", click it and select Allow only contacts on my contact list... done

Thanks! This was the first result I checked from google search for 'msn spam'. The quick hint for Adium has saved me from this annoyance.