Lemmy doesn't tie the images to the resources either, so you can just upload images, get the links and never use it on Lemmy. Often that content is very illegal or questionable.

Time is a surprisingly effective defense, and very cheap. You can hire people to solve captchas or compute for AI, but you just can't speed up time so once the accounts are burned they're burned. And if you try to stockpile them, it's likely some pattern can be identified and ban them all quickly.