Intolerance for the intolerant does not have to be enforced with defederation, and unless their users are actively causing problems outside their instance or they are hosting illegal content i would argue shouldn't be enforced with defederation.

I am of the opinion that the better way of dealing with these types is speaking to them, and challenging their points. you might not be able to convert the ones already there, but you can present the other side of the argument to others who have yet to be sucked in.

They have a grand total of 19 users. it's not like they're not vastly outnumbered. Shutting them off and not challenging their views will only lead to them growing