Taking him down is very simple :D

Buy a hidden camera/recorder, bring up the blackmail in the conversation, report it to the police. For me I would try to make him go as far as to sexually harrass me but unlike me Juhyo's been through some shit so I don't think that'd be a good option. It'd be even better if the camera was connected to some sort of app so Jaemin can stay close by, watching it happen live so incase anything happens he can stop it. All of this sounds pretty expensive and seeing how much that asshole has prepared for this this might backfire but it's worth the shot.