Can't any other modem (on which you have root access) be used to flood it? Furthermore why are they even worried about a modem when someone with malicious intent can just ground the cable or even send mains electricity into it and permanently damage all the equipment on the line?
At least as far as I know, cable-tv networks and devices are protected against both of these failure modes: grounding should trigger a fuse in the amplifier which means at worst it's one's own house that is cut off, sending mains on the line should not be problematic as overvoltage protection is present anywhere across the line because of lightning strike protection, and customer-side equipment should be capacitively coupled only.
Sending crap on intended frequencies that have no protection across the network however is a real problem.