Use a custom DNS and/or hosts file. You can cut them off the grid by blocking data upload to SSP. Don’t install many apps, for games that can be played offline, play them offline. EDIT: AdGuard DNS doesn’t block the 1st URL (o.isx…) in the page. 2nd URL is blocked.

First router has DHCP enabled with range 192.168.1.20 to 192.168.1.80. Second one has “DHCP Relay” enabled with relay server set to 192.168.1.2. Third router has DHCP disabled.