Disabling a facebook website is such a confusion because using https:// still can access it.. what's the effective way?

In ISA server after block https;// you can access it. this traffice come from SSL-Tunnel. you should block SSL-Tunnel on ISA Server.  This article will help you to block SSL-Tunnel ports.  I have applied on my ISA Server2007 now its working perfectly:

http://www.isaserver.org/articles-tutorials/articles/2004tunnelportrange.html