We require an email address and a response to a question on our signups. The response doesn't need to be more than about 5 words, it's just to stop bots putting random characters or single words in there.

So far, it has seemed to ride that balance between low bar of entry and too hard to spam with bot applicants.

That said, if I wanted to spam the Fediverse, I'd just spin up my own instance of Lemmy or Mastodon.