Are there any membership approval plugins where you can ask questions, say, technical questions, to help assess whether a user applying for approval is genuine? Some Facebook groups have this. How effective this is is a different matter, but may sometimes be better than nothing, which appears to the default in Discourse.
You can add some custom field(s) that ask some question that a community member would know the answer to so that whoever reviews new accounts can look at that and see if it sounds legit.
It could be possible to use Custom Wizard Plugin 
or similar to automate it.
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