In the case of legal or law enforcement threats, when a problem has become so severe a lawsuit or police intervention is the best of all bad options - and there are legitimate grounds for filing suit or calling the police, such as blatantly, provably libelous statements made by someone with enough position and money to be worth suing, or threats of actual violence made by someone who has shown ability to carry them out.
Bob is someone capable of such legal or physical harm off the internet (an IP lawyer or a Force Recon Marine on the legal side, an organized crime boss or a Serial Killer on the criminal side for example), but on the internet, he's the most peaceful, self-controlled person possible.
A Serial Killer or the like tries to cultivate an image online that is not only polite, but rather cowardly and wimpy to lull his potential victims into a false state of security.
Bob threatens someone with something other than violence or lawsuits.
Rather than making direct threats, Bob talks about his targets' loved ones (by name) mentioning things about one could never discover on a Facebook page or even casual hacking attempts, and does so on open forums.
The nonviolent, nonlegal threat is something he's actually capable of doing and he actually carries it out.
The threat is used as a form of derailing to change the topic or get people fighting.
Defied: In a situation where making a threat might even be legitimate (e.g. calling the police or the person's parents or filing a lawsuit might actually be a good idea), Bob refuses to make the threat and instead tries to handle the problem himself.
Discussed: "I think our forum should have a rule against posting pictures of personal firearms. We're not a gun forum and this has been a problem in the past with internet tough guys making threats."
Conversed: Two people are having a conversation about the time someone threatened them on the internet.
Implied: The Other Wiki's entire page on its No Legal Threats policy, which implies that, yes, in the past people have made threats of legal action to silence discussion - and that is why legal threats are a bannable offense there.