Just as an example of 'not cricket', my daughter recently had very good reason to block an instagram user. Except, if that user is in any groups that you're a member of, in order to still see group messages, you must accept to still see their messages (and they yours) in those groups - or you must leave the group.
Actually being able to block other users, whether they're in a group you're in, or not, does not seem like an insurmountable technical challenge to me, so I assume it's a policy 'nudge' to discourage blocking toxic people.