Cloudcodes gControl Plus od gcontrol

This is an extension to gControl for gmail block.

gControl is advance access control for Google Apps, gControl provides the admin an additional set of controls to meet the enterprise needs of access control of Google Apps on the Cloud.

Using this extension, the Google Apps admin can block its user from accessing his personal Gmail account in the company network.
The domain needs to use gControl in order to use this extension. The admin can force its user to install this extension by enabling Gmail Block settings in gControl Settings.