Is there a Discourse dialog standard or custom with a similar type of this screening dialog? It might be a very good way to discern bot vs not - for now.
"To join this server, you need to fill out this application, and wait to be accepted.
To combat brigading, we have restricted user registration on this instance. Please write a short description containing:
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Why you would like to join XXXX
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What communities you would most like to participate in, and
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How or why you chose the username you did.
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Copy and paste any full sentence from link.
We use these questions to screen for and discourage spammers and trolls. We will try our best to review your application as soon as possible."