This article describes how to block/unblock devices from connecting.
INSTRUCTIONS & INFO
From the home screen, tap Wi-Fi. Note: 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. For GoPhone Users: Your device will have a different home screen showing ‘To check your data balance, go to att.com/mygophone’
Scroll to, then tap Connected devices.
Tap the desired connected device.
Tap Block. Note: Tap 'Unblock' to unblock the connection.