User.joins(:user_stat).where("user_stats.post_count = 0 AND previous_visit_at <= '2016-05-22'::timestamp").destroy_all
Should do the trick 
Of course make sure to make a backup first
Edit:
Sorry, forgot usage tips Of course you have to log into your server, go into the discourse directory [probably /var/discourse], enter your Docker container [probably ./launcher enter app] and then enter rails console by typing rails c . These are defaults on docker based installations
@treb0r
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