Spam by MSN?

Hello,
I do not know where exactly post this problem because I do not know the name "spam by MSN" perhaps?
My PC is switched on 24/24 and I have about 10 times a day MSN Popup window that opens with a name on phony, bogus email address, and a institutes in English for fake diplomas from prestigious universities And an invitation to call a phone number in some part in State. Needless to freeze in contacts MSN because it is never the same email address.
Google is not my friend on this one here, would you please help me in this?

Thank you in advance.

In options, there is something that defines whether messages contacts that are not on your list are accepted or not:

If you have MSN dial-up Internet service, you should upgrade to the most recent version of the dial-up connection software. This version fully supports Windows Firewall in Windows XP for MSN dial-up Internet service users. For more information about upgrading your MSN software or securing your MSN Internet connection, contact MSN support.

Thank you for your reply Tweaker, but unfortunately I do not Windows Live but pidgin (which includes several messengers and manages several e-mail addresses), and there was not this green list.
In addition I find this solution too radical because you can very well, in a forum, leaving the possibility for other members of Instant Messenger to contact you, without being obliged to enter all members in your green list.
Too many security interactive kills!