Please. Captcha by default. Email domain filters. Auto-block federation from servers that don't respect. By default. Urgent.
And yes, to refute some comments, this publication is being upvoted by bots. A single computer was needed, not "thousands of dollars" spent.
No, you've (maybe) limited your singular solitary instance's growth: your instance is not "Lemmy" and admins should do whatever they find works for them, is something they can easily enforce, and resolves the problem.
If you want to geoip limit signups to Skokie, Illinois? Great! If it works for you and keeps your instance from being The Problem, then it's a valid solution.
(I don't disagree that email domain blocks are not a singular solution to any abuse problem, but I also think that whatever works for the individual admin is perfectly reasonable, and email blocks CAN be worthwhile.)