Yes, you can unpair a mobile device specific to a user but its important to realise that they can pair it again unless you remove them from the audience that is permitted to use the mobile application.
To unpair an individual users device you would click on Actions > Configure > Manage Users > Devices. You will then see the following screen.
You can then use the 3 dots to unpair the device or click on 'unpair all' to remove multiple devices from this user if more than one has been connected.