Neighbour Keeps Accessing my Network/wi-fi despite password changes - How?

I've noticed a device on my network that belongs to my neighbour, and no matter how many times I change the wi-fi password, they keep getting in.

I've already:

Factory reset router Changed SSID and password multiple times (using WPA2)

In the connection type is says disk, I'm assuming this is somehow related to a WiFi disc extender. I have no WiFi disk extender.. I only have the router a BT smart hub 2.

I've called BT and they've been no help, they seem to know less about routers then I do and I don't know anything.

How can they still be connecting? And what can I do to stop them permanently?

Any help appreciated.