Mobile:

1. Before you continue, delete your offline content.
2. Tap on 'My Collection' in the lower right of the app. 
3. Tap on the wheel icon in the upper right of the screen.
4. Scroll down to 'Authorized Devices' and tap on it.
5. Tap on the device you wish to deauthorize.  
6. Tap on the 'Deauthorize Device' button.

Desktop:

1. Go to account.tidal.com and log in.
2. Click on Offline Devices.
3. Select the device you wish to deactivate and click on Remove Device.