Step 1:
Use a spare modern or WiFi access point.
On this device, create an SSID with the same name and password as your current setup.
DON'T connect this device to the internet. It ain't connected to nothing!
Step 2:
On your main router, change the SSID and password to some secret stuff.
Connect all your and your family's phones, tablets, etc to the new SSID
All done. Now you and the family can continue happily. The outlaws and the Dom will connect to the dummy WiFi, as they already have the SSID and password programmed in. When they complain of no internet access, scratch around a bit, then say you can't find any fault. If they get narfy, scratch some more and "accidently" factory-reset their phone. No more problems...