You can restrict the forwarding address option by using following steps.
To disable automatic forwarding:
Sign in to the Google Admin console.
Do one of the following:
In the classic Admin console, click Settings > Gmail.
In the new Admin console, click Google Apps > Gmail > Advanced settings.
In the Organizations section, highlight your domain or the organizational unit for which you want to configure settings (see Configure email settings for an organizational unit for more details).
In the Automatic Forwarding section, clear the check box Allow users to automatically forward email to another address.
It can take up to 30 minutes for the change to take effect