What works for me on both mastodon and Lemmy is a free text question: why do you want to join?
The user enters whatever they like and it goes into a moderation queue. Both lemmy and mastodon send me an email when a new account is ready to review.
I read the response and choose to whether to approve their account. At the moment, spammers are really bad at answering the “why do you want to join” questions.