The admins of those instances could just look at the DB, and DROP all the users who post nothing. Those IP addressees could also be added to a shareable ban list. CAPTCHA needs to be added to sign-up without delay. I’m sure the experienced Lemmy admins can help out the newcomers with this. Pruning of the tree needs to begin now, before it becomes too busy to make large deletions to the DB on-the-fly.