In theory, newer powerline adapters can use encryption to isolate networks. As usual with such protocols, the devil is in the implementation details.

There appears to be no affordable physical (L1) firewall for powerline networking. There are "EMI power filters" in the 14khz to 40Ghz range for hospital MRI rooms (https://www.astrodynetdi.com/mri-shielded-facility-emi-filte...) and mil-spec SCIF isolation, with starting prices of a few thousand $.

MoCA offers affordable ($10) devices to block ethernet traffic between your home cable modem and the external coax, e.g. https://www.ppc-online.com/products/coaxial/filter-SNLP-1G70... & https://www.amazon.com/gp/product/B08HJ4F4D4/