My question would be why do you want to stop it being possible to setup users?
To use that app, or any other app which has Device admin powers you grant it ownership of the device and it can control what you can and can not do on the device.
In case it is not clear I will state it very clearly. Granting Device admin to any app has major security implications. Granting an app the ability to act as an Accessibility service has similar implications. Using Auditor app you can robustly check to see if either of these abilities has been granted to any app on the device.