http://www.plevna.f9.co.uk/blockchat.htm
các bác tham khảo thử xem coi sao:
"block Trillian - Trillian connects to other messenger (yahoo, ICQ, AIM and MSN) so you must block those ports - 80, 1863, 5050, 5190 and a dynamically allocated port) to successfully prevent Trillian. You will not successfully block port 80 and an unknown dynamically allocated port and still get web site access in Internet Explorer. TerminatorX can solve the problem."
hay ở http://www.comvigo.com/blog/?p=27
nếu ở client xài proxy để vào yahoo chat thì bác có chặn cũng thua à... em nghĩ bác nên cấm quyền cài đặt trên máy client luôn. vậy là ok