I'd really like to be able to install something on the lines leaving my breaker panel that acts like a firewall and blocks any kind of network communication over powerline.
1. Get some electrical-noise suppression ferrite toroids and some ceramic capacitors at your local electronics store. (.005 microfarad at a minimum of 600V would be good for the caps. 1000V or higher on cap used for 240V circuits.)
2. In your fusebox connect a cap from each breaker's hot output to the nearest ground bus, keeping the wires as short as possible. (You want them downstream of the breaker so they blow the breaker, rather than start a fire, in the very unlikely chance that one fails shorted.) On 220v loads hook the cap between the two hot wires (red and black in the US).
The cap wires are too small to carry the current in case of a short, so get some tiny 1A pigtail fuses and wire them in series on the hot side (either side in the 240V both-are-hot case). Put plastic insulation rated at least 600V over (at least) the hot side wires and the fuse. (You can get such insulation, of adequate voltage and temperature ratings, by stripping the insulation from a spare piece of electrical wiring.)
3. In your fusebox disconnect the circuits, one by one, both hot and neutral. On each run the hot and neutral lines through a ferrite toroid core in opposite directions and reconnect the . For a 220V circuit run the two hot lines through the ferrite core, again in opposite directions, and ignore the Neutral. If you have multiple loads on a breaker, you can use separate toroids on each load or a single one on from two to all of them: Run the hot wires all one way through the toroid and the neutral (or red-hot on 240V) the other way.
This puts inductance in series with the signal and capacitance shunting it, forming a low pass filter. The low-frequency power will get through just fine and the high frequency networking signals will get stopped.
Putting cores on the main feeds also works, and takes fewer cores. You can also put one on each of the hot wires, separately, rather than using one with the wires crossed through it. You can get big ones that are split, intended to be clamped over a computer signal cable to prevent it from acting like an antenna, which you can clamp onto the wires without unhooking them.
Don't bother putting the two haves of the circuit through the core in the SAME direction, as you would with the signals in a cable or power cord if you clamped a core around the whole thing. This keeps the common-mode (both wires go positive or negative together) from propagating past the core, but the differential mode (one goes positive while the other goes negative), which is what power line networking uses, goes right through.
Note that putting this stuff in your fusebox may be against code, and void your fire insurance. The capacitor wiring may also be problematic for creating hazards if not done properly (insulated with "spaghetti tubing" on at least the hot side, hot side cut short, a little fuse (1A or so) in series, etc.).