If you wish, you can force your users to use the push-to-talk hotkey. In order to do so, follow the steps below:
1. Go to Settings -> Options -> Application -> Check "Advanced permissions system".
2. Go to Permissions -> Channel Permissions.
Note: Make sure that no "filters" are selected at the top of this window
3. Click Client -> Modify.
4. Double click "Force Push-to-Talk"
5. Ensure the box is checked (which makes it 'enabled'):
Please open a support ticket if you have any further questions on this subject.