This article describes how to block/unblock devices from connecting.
INSTRUCTIONS & INFO
From the home screen, tap Wi-Fi. Note: A Wi-Fi enabled device must first be connected to the AT&T Unite Pro before blocking/unblocking it. Whenever you block or unblock devices, the Wi-Fi radio resets to activate the new settings. Any devices connected to the mobile hotspot will be disconnected when the Wi-Fi radio resets.