I think the flat-Earthers are conspiracy nuts along with the JFK conspiracy nuts, the moon landing conspiracy nuts, the Pearl Harbor conspiracy nuts, etc., and I think they are all deluded, but how are flat-Earthers causing ANY harm to society, never mind serious harm?
They have thoughts out of sync with 99.8% of the rest of society (if the Denver Post is to be believed). So what? They're not robbing banks. They're not beating up astronomy teachers. They're not sabotaging globe-making factories.
They're renting conference space and sharing their ideas with others.
Except for the janitors who have to clean the rooms afterwards, what serious harm are they causing?