This community does have established and explicit rules, such as "no proposing violence, not even hypothetically".

Those rules are not in a place where a new member would easily find them. Some people even think there a rule against politics when there's no such thing on LW.

Someone who engages in block downvoting would be sending a signal, using tools as were provided. What's the obsession with the punishment-concept?

You were the person who started speaking about punishment. For my part when I was forum moderator I didn't think of myself punishing weeds when I tried to rip them out of my healthy garden. I did ban people but not to punish them but because I thought the forum would be healthier without them.