Only the Owner(s) can modify access rights. In the Somfy Protect application, go to 'Users', then click on the relevant user. Their profile will open, click on 'Delete'.
Note that you also have the option to temporarily block access to Door Keeper (and other controls of your installation if you manage multiple devices) for your users. To do this, simply toggle the slider to the left (it will turn gray) from the list of users.