If a staff member gets a new phone or device you can revoke access to the mobile app on the old device. This can also be used if a staff member moves on to a new school, though you should be sure their account is disabled as part of your exit procedures.
You can revoke access under the Devices section of Administration. Select Approved see a list of approved devices for your school. Find the device that needs access removed and click on Revoke. You will receive confirmation that this has been done.
If you have accidentally revoked a device you didn't intend to, you can re-instate it by selecting the Revoked list and clicking Approve.