From my dark archives, here's a list of ports which you could start blocking if you want the respective services to be stopped. also do remember, if you want your web access to be operational, you should refrain from blocking port 80, a well known port for HTTP.
MSN Messenger
Service: remote: 1963 and 1863 local: 1542 and 1493.
Audio: local: 13803, 1556, 11771
File transfer: remote: 6891 local: 1544
Messenger Update: remote: 80 local: 1457
Sign-In: remote: 443 local: 1484, 2400
Remote Desktop: local/remote: 389, 522, 1503, 1720, and 1731.
ICQ/ICQ Lite
Service: remote: 20 and 5190 local: 1076
File Transfer: remote: 1102, 2431, 2439, 2440, and 2476 local: 1101 and 1102
Yahoo! Messenger
Service: remote: port 5050, 5150, and 8000 local: 5101 and 1638
Audio: remote: 5001 local: 1644 and 5000
File transfer: remote: 1614 local: 80/81/82
Webcam: remote: 80 local: 1677
Y!Tunnel
Service: remote: 1455, 5050, 1071, and 1074 local: 8000, 8001, and 1073
AIM
Service: remote: 5190 local: 5180, 1330, 1335, 1338, 1344, 1339
Audio: remote: 2773 local: 2564
File Transfer: remote: 2153 local: 4443
Sign-In: remote: 5190 local: 5180
mIRC
Service: remote: 6667 and 6660-6666/6668-6669 local: 2367
File Transfer: remote: 3697 local: 1107
Ident server: local/remote: 113
Picking pebbles on the shore of the networking ocean
