anonymity and privacy seem to come at odds with a social platform’s ability to moderate content and control spam.
If users have sufficient privacy and anonymity, then they can simply use another identity to come back, or use multiple identities.
Are there ways around this? It seems that any method of ensuring that a banned user is kept off the platform would necessitate the platform knowing information about the user and their identity
I agree that this is good, but depending on your threat model, this may not really be sufficient anonymity.
Post and comment history may be plenty to identify you, and your IP and other personal data can still be visible to instance admins.