Extensions can not be unregistered by restarting your PBX. You either need to unregister it by SIP protocol (e.g. when closing your Softphone) or by waiting registration timeout to expire.
On mobile extensions you have the option of selecting "WLAN_Only". This makes registration timeouts much shorter. Some devices and softphones allow you to configure your registration timeout. Registration timeouts should not be less than 3 minutes.