Internet

Most routers have access controls built into their firewall.  What router or gateway do you have?  You may be able to block specific devices from accessing your network.

Also, if they are connecting via WiFi, someone there must have given your guests the password.  You can change the password and not tell them.  Drawback is your will have to change it on all your devices.  If they are wired directly into your router/gateway, you have to go the access control route as there is no password on a wired connection.

You can authorize only certain devices to connect to your WiFi router.  It's MAC Address Filtering.  Meaning, you enter all MAC addresses of your devices...such as TV, Roku, laptop, cell phones, tablets, etc...and your router only allows those devices to connect.

Also...it's a good idea to change password...technically named SSID (Service Set Identifier).

Bruce said:

Also...it's a good idea to change password...technically named SSID (Service Set Identifier).

The SSID is the network name, most routers/gateways default to broadcasting the SSID but you can turn that off.  The password is also called the Network Key or Pre-shared Key (PSK).  I agree, it's a good idea to change it.

I meant to write "encryption key for your WiFi network" but, you're correct, SSID is the name of your wireless network.