Cannot sign out of iCloud. Photos app not updating

I am trying to sign out of an iCloud account but the Photos app is not allowing it to happen.

I am going in via System Preferences > iCloud and click "Sign Out". I then confirm by clicking "Delete from Mac". I then get a message saying:

"Open Photos to disable iCloud Photo Library

The Photos app was recently updated. Open Photos to update your library before disabling iCloud Photo Library".

I then click on "Open Photos...". At this point Photos opens but nothing appears to happen. This is confirmed by trying to sign out of iCloud again and seeing the same messages.

Things I have tried already with no success:

1. Rebooting machine
2. Checking for Software Updates
3. Forced repair of the Photos library (by Command-Option clicking on Photos app)

Does anyone have any suggestions for what to try next?

Thanks