You need cryptographic identities for automated boycotts. Boycotts aren't very effective today because they're hard to organize, and some vendors have relative monopolies. I think Ethereum enables both effective boycott coordination and monopoly elimination.
Also, these sanctions apply to both sellers and buyers. I think economic isolation will be an effective deterrent to undesirable behavior. It's basically prison, just without force.
Imminent harm may be a slippery slope, but it's really all up to the individuals who support the enforcers. If someone detains someone who shouldn't have been detained, stop funding their activities and sanction them. Fund a different enforcement organization instead.
I can see the boycott coordination angle (except, the offensive vendor can just change its identity).
By what mechanism does Ethereum enable monopoly elimination? (sorry, I don't know too much about Ethereum)
The problem with detainment, collective detainment, sanctions, etc. is that the local conditions when a detainment occurs may be far from the theoretical situation. What if everyone around you believes that you should be detained, but you actually shouldn't (according to the theoretical 'rule')? You just threaten to "stop funding" them and they let you go? What is to stop them from escalating the violence? Nothing.
"I can see the boycott coordination angle (except, the offensive vendor can just change its identity)."
If the vendor changes its identity but the public is still aware of the connection, the new identity goes on the sanction list. If the new identity can't be linked, then it has absolutely no reputation and has to start from scratch to convince people to buy from them. That is an effective deterrent.
"What if everyone around you believes that you should be detained, but you actually shouldn't (according to the theoretical 'rule')?"
There are no "rules". What I'm describing is a society organized via voluntary actions, but by following prescribed rules. If everyone around you thinks you should be detained, you get detained. That's how society works.
Sanctions don't prevent bad behavior, they punish bad behavior as a deterrent. Separately, a police force exists only as long as people fund it, and as long as people approve of their uses of force.
I'm not saying this is all bulletproof, but I think it's going to be fun to explore as we build the foundations for these sorts of systems.
I'm not sure I understand why I need a cryptographic identity to boycott certain vendors? I can already do that today.