Let's stop with the reference class tennis. This community does have established and explicit rules, such as "no proposing violence, not even hypothetically". It is not like one of your parties, I suspect. And while you may tell someone to leave you alone, or to get out, I wouldn't say that official punishments against breaking inofficial "norms" are the rule. At least hopefully nowhere I'd like to be. Note how this community has grappled time and again with coming up with a clearly defined norm on this, which would decohere the congruence even if LW were like a party. Meet-ups have resorted to clip-on notes whether hugging is ok with that person. So much for implicit norms for social interaction.

Someone who engages in block downvoting would be sending a signal, using tools as were provided. What's the obsession with the punishment-concept? (Warning, flippant aside: Do we need a good public beating, or what?)

If it isn't applicable then what's wrong with TDT?

In a word, nothing. If you use TDT going off of "I don't want agents to be punished for actions against which there are no rules", then TDT will include that when giving you the optimal course of action. If you use TDT going off of "I don't care whether agents are punished for actions against which there are no rules", then it won't include that. It's the reason why paperclippers and anti-paperclippers both can use TDT. TDT doesn't judge your preferences :-).