Fuc.ing onDisconnect method

Hi,
I need a help. A whole day I am searching with my friend, called Google, for any good idea, how to do onDisconnect method working. If I unplugged a net cable or client is behind a firewall "onDisconnect" method doesnt recognize, that client was going out. I try to use a ping function, but with no result. Now, I dont know how to continue. It is very important thing for me. Can you help me somebody? :-(
Honza

Sorry for my bad english...