As Admini.,how can i block my staff in using yahoo messenger

As a system administartion of a company of about 40 computers.I have been given an authority to deny access to some member of staff in my ofice from using yahoo messenger.what can i do and how do i do it?

You could try blocking cs1.yahoo.com, cs2.yahoo.com, and cs3.yahoo.com.

We have just taken that decision ourselves (a unilateral decision by myself and the other network/security guy).

We run ISA 2000 as our firewall and it has the option to deny specified programs from accessing the internet through the firewall. Therefore we just specified the .exe of some of the more common IM applications (AOL messenger, Yahoo messenger, msn messenger, ICQ etc)

If you run a Windows based network and use Active Directory you can use a group policy called "Software restriction policy". But from what I understand the clients must be running Windows XP.

Hope this is of some use

p.s. Also bear in mind that many of the Instant Messaging applications can run over a range of different ports, including port 80 so blocking the destination or the app may be more successful than the port.

We run ISA 2000 as our firewall and it has the option to deny specified programs from accessing the internet through the firewall.

How does the firewall know the programs name who is accessing the internet?

With ISA 2000 you have a firewall client that sits on the user node.
The firewall client is not necessary for usernodes to be able to access ISA but if you deploy it you do seem to have more administrative control.

If you go via ISA management and browse to
Servers and Arrays | <Server name> | Client Configuration
and then double click the firewall client you can go to the Application Settings tab and put in rules for specified .exe's to disable.