No one questions that segregated groups with tight information and communication control can result in more efficiently reached results. History has shown us several examples of such highly efficient societies where what can and cannot be said where tightly enforced. But history also has shown us how such societies eventually break from unresolved conflicts within it.
Talking to someone with another viewpoint, even if that viewpoint is a stupid or hateful one, does not mean endorsing. This is not about "fair and balanced", it is about communication and providing ideas outside of their peer group. Now, if you decide to just block flat earthers individually, or as a group (even for non-flat-earth-related communication), you will just create a filter bubble for them in which they can strengthen their now-unchallenged beliefs in.