I have a Google Docs document that I allow public comments on. Lately, one specific user has been repeatedly adding unhelpful comments and hiding (marking as resolved) all of everyone else's comments. Is it possible to prevent just him from interacting with my document further, without removing comment access from the rest of the public?
No, this is not possible. If a document is set on public commenting then it's transparent to all users. This would be possible only if you would make it private and add email addresses of people you want to comment - then you could just ban a specific user by removing his access to comment or fully remove him from permissions to view your document.