@Waverunner1 

In my Deco App on the phone I can add devices to the BLOCK LIST (after they connect), either by MAC Address which can be changed depending of Privacy addressing, or by device Name (which could also be changed easily). You can also enable CONNECTION ALERTS for NEW Devices, then manually go in an block them.

In the Deco App on the phone under WiFi Access Control there is a Block List Mode.

2 choices, only devices NOT on the Block List will be able to connect OR Allow List Mode, only devices on the list CAN connect.

I know the above is on the Deco app, not sure if on the Tether app but would expect to be.