Something

acl msn_url url_regex -i gateway.dll
acl msnmessenger req_mime_type ^application/x-msn-messenger$
acl msn_port port 1863
acl msn_method method POST

http_access deny msnmessenger
http_access deny msn_method msn_url
http_access deny msn_port
http_access deny CONNECT msn_port

acl MSN_Messenger browser ^Mozilla.compatible;.MSN Messenger.
http_access deny MSN_Messenger