Google's John Mueller said you can ignore threats and blackmail emails from people who demand do-follow links. John said on Reddit when asked about these threats "ignore it, it's just spam."
The email received by this person contained this threat:
...If I do not seeing a backlink in one week, I am create million toxic blog comment spam and redirect backlink to codeforest.net and you can saying goodbye to your Google rankings for 1 year or more.I trust you making correct decision...
He asked, what can he do about it?
I assume most of you who get these emails, those emails get caught by spam and you do nothing about it anyway. But if you do get those emails, just delete them and don't worry about it.
At least, that is what Google recommends and has been for years and years.
Forum discussion at Reddit.