You can buy valid gmail address by the thousands. Email validation is one part of a multilayered approach. It cuts some out, but you need more layers. Captchas work, they cut some proportion out, but not all.

Probably the most effective is registration applications, but this is a huge barrier to entry. If we want Lemmy to grow, we are going to have to change the current state (most instances require an application to join), or change peoples expectations. You can sign up for a reddit account just like that, and start using it without waiting for approval. Why would people choose Lemmy? On our instance we had a drop in registrations to about 1/10 of what we had with open registrations.

Unfortunately I don’t know the answer. It probably involves taking on strategies like reddit if we are going to scale that big (auto-mod, karma, etc). Unfortunately we will have even more trouble, because in the users host instance doesn’t ban them then an admin on every other instance has to ban them for that instance. So we probably need to be able to follow ban lists to auto-ban users that have been banned on other trusted instances or something like that. As we grow, I’m sure we will have more pain before it gets better, but I’m hopeful that we will solve issues as they arise.