Configure your internal DNS to return 127.0.0.1 for:
Also add the above DNS names in the Web proxy server black list.
To make it more effective:
Google chat uses the following ports and servers for it's chat service. Ports (80, 443, 5223, 5222), Servers (18.104.22.168, 22.214.171.124, 126.96.36.199, 188.8.131.52)
Create two Alias and club all the ports together in one and the IP addresses of the servers in another Alias. Now create a rule in Mettle SE for the local networks where in you block all the traffic from LAN to google chat servers on the mentioned ports. Use the Alias you have created in the firewall rules. To block only google chat file transfers block the ports 20 & 21.
You are recommended to allow only known HTTPS web sites from your LAN through the Web proxy server. This can be done by entering "**s" (without quotes) in the Web proxy server black list and then add the known sites to the white list.