The jurisdiction that the forum operates in is the US. Blackmail is illegal there, as defined by
Whoever, under a threat of informing, or as a consideration for not informing, against any violation of any law of the United States, demands or receives any money or other valuable thing, shall be fined under this title or imprisoned not more than one year, or both.
(confusing part bolded). There are legal frameworks for taking down infringing content, but I do not know of any for taking down blackmail. However, if you believe you are being blackmailed (according to the above definition), it's always worth a shot to send theymos a PM.