Colin McGinn (Miami) implies that he may pursue certain individuals for libel. While the UK is at one extreme on libel (very friendly to victims of alleged libel), the US is at another (very friendly to those who allegedly libel, indeed, to those who would be found liable for libel in almost every other Western democracy). When it comes to "public" figures like McGinn, it is not enough that the claims be false and damaging to reputation (though they must be at least that), they must also be made "maliciously," that is, with reckless disregard for whether or not they are true. That is a quite difficult evidentiary burden to meet, which is why "public" figures (a capacious category, as it happens) in the U.S. usually have no meaningful legal remedies for pernicious false statements of fact about them. And since truth is a defense, any libel action also opens the door for the defendant to engage in wide-ranging intrusions into any part of the plaintiff's life possibly relevant to the truth of the statement. There may be actionable statements about McGinn out there, but the likelihood of that is low, given American law.
UPDATE: Bill Edmundson, a lawyer/philosopher at Georgia State writes: "If McGinn is classified as a public figure (as I agree he probably would be), then under Philadelphia Newspapers v Hepps he has the burden of proving falsity -- in the US it is no longer the defendant's burden to prove truth (as is the
case with affirmative defenses generally). ). This, on top of having to prove actual malice by clear and convincing evidence." Professor Edmundson adds: "One wrinkle has to do with whether Hepps applies to matters not 'of'public concern': but what's been said about McGinn is presumably of public concern. (Despite dictum suggesting the contrary, the status of the defendant as "media" or "non-media" is probably not of constitutional significance (cf. Citizens United).)"
ANOTHER: Just to clarify in response to some reader e-mails: Opinions, even if damaging, are not actionable. So, e.g., the judgment, "Graham Harman is not very good at philosophy" might damage his reputation, but it would not constitute defamation, since it is "merely" the expression of an opinion (libel is written defamation, slander spoken). "Graham Harman is a bully" is a harder case: on the one hand, the accusation of bullying seems to imply something factual about the individual's actual conduct, but, on the other hand, in our era of cyber-cry-babies, the label "bullying" is so widely abused (for reasons usefully elucidated here) that it increasingly looks like an opinion ("I don't like that Graham Harman is mean to me"), not a shorthand for a factual statement about conduct. Finally, "Graham Harman is a convicted felon" would be libel per se (i.e., one would not even have to prove damage), on the assumption (which as far as I know is correct) that it is a false statement.