The TOS is a contract. If the poster has "defamed" someone within the meaning of the contract, then DO is within their right to terminate. Moreover, the TOS als

The TOS is a contract. If the poster has "defamed" someone within the meaning of the contract, then DO is within their right to terminate. Moreover, the TOS also gives DO a lot of discretion to make their own determination of what constitutes defamation. It is not vigilantism because DO is not threatening to throw the poster in jail over an allegedly illegal act. They are simply enforcing their contractual rights. You could argue that this is a breach of contract, and that the poster must be shown in court to have defamed someone before the contract is terminated, but note that the section of the TOS that DO cites also says that the poster cannot "harass or embarrass" someone. That's pretty vague and gives DO a lot of leeway to argue that the poster breached the TOS. It's a pretty crappy TOS to be sure, but I wouldn't bet money on the poster winning a court case here.