I don't like being a parade-rainer-onner, but as I mentioned recently in another thread, there's some issues with the idea of being asked to be put on the DNC list:
1. The instruction coming down from COs has been to not put someone on the list just because they ask you to. Yes, you heard me. We're [I'm still an active Witness] supposed to ignore the request. You would have to come to the door with a revving chainsaw and hockey mask to persuade them to mark you as a DNC. Or, be an apostate. Some here have suggested that householders be prompted to say "I'm an apostate" in order to get the DNC status, though personally I'm skeptical of the effectiveness of this.
2. Elders still [are supposed to] check on the DNCs once a year to make sure that the person currently living there still wants to be a DNC. That's already as often as some people get the JWs at their door (at least while they're home).
3. Depending on the way a congregation manages their territory cards, and how flaky the specific car group is, DNCs can be frequently ignored altogether. "Oops, I forgot to check the back of the card. Well, we already worked all the DNC doors. Oh well."
I'm not saying that people shouldn't be trying to give an anti-witness to warn people about the religion; far from it. But I just don't see this particular approach being very effective. If people expect that saying the words "Do Not Call" will magically stop JWs from coming, they will be sorely disappointed.