Honestly its rude to be invited by a country to meet their president who decided to respond to your criticisms of their administration only to not show up and go meet the opposition.
Being invided and then no showing shows complete disregard to basic diplomatic Etiquette since it was a mission to show solidarity against the embargo
Bonus socdems being cringe part 4.5: about another member that didnt show up to the presidential meeting
Deeply unserious people
it is pretty common for people in orgs to get involved, it is something that should be planned for
Genuinely what would you suggest?
it seems like in this case the abuser was using people as accomplices, otherwise the response from the local org wouldn't have been what it was.
Yes; there was no mechanism to go around local leadership. Now there is.
How?
Explain the policy, talk a great deal about comradely behavior and expectations of conduct, and do our best to get their vibe. If that's not sufficient I would again ask for a concrete suggestion.