Making stupid comments isn’t a problem, posting stuff that is against the instance’s rules is. So if they’re harassing people or posting disallowed content or something consistently (more than a handful of individuals), and the admin either refuses or ignores requests to take action, then they should be defederated.

If you have an articulable set of problematic posts that demonstrates that it’s not just one a handful of users, post those and we can discuss defederation. If it’s urgent (e.g. the instance is under active attack), bring it to the admin’s attention immediately.