You can’t delete (or add) users directly in SignatureSatori. In order to remove users from the app, you need to delete them in G Suite and then click “Load new users from G Suite” in the Users menu.
You'll be now prompted to confirm your choice.
The synchronization will now be performed, new users will be added and the ones deleted in G Suite displayed above the list of Users awaiting your confirmation:
Voila, the user's gone!