Another good answer, I remeber from my old TV days that they used to put a collection of capacitors on the RF signal just as it entered the TV for isolation, this was probably to shield the ariel system from mains volts. I wonder if there is any type of sytem isolation employed, we are all on very long piece of wire and you never know what people get up to on there systems behind closed doors!!!
The problem with system isolation is that it has to be selective (like a firewall) as all communication is encapulated in comms protocols so a simple circuit couldn't do it. You couldn't just remove all signals over a certain level.
Any command sent to interegate your box by your supplier would be included in the other constant stream of packets and would need decoding before its characteristics could be determined. To do this in real-time would require some sort of processing. The only way your going to get this is via a bespoke IPS (Intrusion Prevention System) specifically designed for this purpose. Most of the shelf IPS's are IP network based but you'd need to be able to:
1) distinguish the characteristics of the interegating packets to filter them out.
2) Have some sort of ADC provider side conversion and DAC cable box (for digital/analogue conversion).
3) Physical layer conversion to/from ethernet-co-ax.
All this can be provided on your box if you know the answer to 1) with an integrated firewall. I think this the best bet rather than a bespoke device.