more inforamtion is required. Are they using a GTALK client or a web application? Is the traffic going through MWG or is the client able to find a way around MWG because the GTALK ports are available to the users?
You should have a look at the access.log and try to access GTALK and see what records are written to the log. Then block the category or URL to avoid users using GTALK.
In case the traffic does not arrive on MWG and in the logs, you should check the firewall setup and block the ports used by GTALK.
Thank you andre, clients are using Gtalk client. traffic is going through MWG. when they have access to gmail they can easily access google chat, but GTALK is a different utility and it is client just like MSN messenger and yahoo messenger.