Just in recent memory, saw someone claiming to have Game of Thrones episodes that weren't leaked on the pirate bay, now some moron was trying to blackmail Agora by claiming he had their private key. I mean this stuff is funny and harmless, but seems off topic.
DOXXing is already against Reddit's ToS and will lead to posts getting pulled and accounts shadowbanned.
The cases you describe are more trolling for attention than blackmailing for satoshis. In the Agora case it quickly became clear the OP didn't have access to the Agora private key. And while the Grand Wizard didn't follow the Game of Thrones thread he would note that HBO's team of copyright lawyers are quite capable of fending for themselves and don't need this subreddit's protection.
tl/dr; the rules in place will cover actual events of blackmail. Ignoring or mocking trolls as you see fit will handle the rest.