Follow the steps to block Gtalk using Microsoft ISA 2004.
Step1: Right Click on 'Firewall Policy'-->New-->Access Rule.
Step2:Access Rule NameGive the name to particular access rule. I have given 'Block Gtalk'. then click on 'Next Tab'.
Step3: Rule Action. Select the option 'Deny'.(it is default option). then click on 'Next Tab'.
Step4: ProtocolsSelect the option 'All outbound traffic'. (this is also a default option). then click on 'Next Tab'.
Step5: Access Rule SourcesIn the 'Access Rule Sources' pane, we have to add the traffice source. which is actually the 'internal network'. For that
Click on the 'Add Tab' on the 'Access Rule Sources' pane. Then 'Add Network Entities' windows will come. Click on the '+' sign near to 'Networks'. Then select 'internal'. Then Click on 'Add Tab' and 'Close'.
Step6: Access Rule Destination.Here we have to create three custom 'Network Entities'.
1.Computer Set
2.URL Set
3.Domain Name Set
Click on 'Add Tab' on the 'Access Rule Destination' pane. Then the 'Add Network Entity' window will come. Click on 'New' option, the drop down will appear. Click on Computer Set.
'Computer Set' window will come. Give the name 'Gtalk Servers'. Click on 'OK Tab'.

Like wise click on option 'URL Set' in the drop down window. Give the name 'Gtalk URLs'. Click on 'OK Tab'. Finally click on option 'Domanin Name Set' in the drop down window.Give the name 'IM Messengers'. Click on 'OK Tab'.
Computer Set - Gtalk Servers
Here we have to enter some values. For that Click on 'Add Tab' in the 'Gtalk Servers'. Then click on 'Computer' option.Then enter the name IP1 and give Ipaddress respective to this.IP1-72.14.203.125IP2-216.239.37.125IP3-72.14.253.125IP4-72.14.37.125IP5-74.125.65.125IP6-209.85.137.125IP7-74.125.155.125IP8-209.85.229.125IP9-64.233.169.125IP10-74.125.47.125IP11-209.85.163.125IP12-216.239.37.125
URL Set - Gtalk URLsWe have to add urls in the 'Gtalk URLs'. For that click on 'Add Tab' and enter all the urls which are given below.*.gmail.comhttp://chatenabled.mail.google.comhttp://etherx.jabber.org/streamshttp://filetransferenabled.mail.google.com/*http://talk.google.comhttp://talkgadget.google.com/*http://talkx.l.google.comhttp://www.google.com/talk*https://talkx.l.google.comhttps://talk.google.comtools.google.com
Domain Name Set - IM MessengersHere we have to add domain names. For that click on 'Add Tab' and enter the domain names given below.*.talk.google.cometherx.jabbber.orgl.google.comtalk.google.comtalkgadget.google.comtools.google.com