My WiFi access point and router offers a feature that lets me limit access by “foreign” devices: MAC filtering. It’s not perfect, but it stops the unsophisticated device user from latching onto my Internet bandwidth. That and a WAP password.
MAC Address filtering is a pretty good way of limiting access if your list of ‘approved’ devices is fairly static. Otherwise, you can end up spending a lot of time modifying your ‘allow’ list.