To be defamatory in Maryland a statement must “expose a person to public scorn, hatred, contempt or ridicule, thereby discouraging others in the community from having a good opinion of, or from associating or dealing with, that person.” Batson v. Shiflett, 325 Md. 684, 722-23 (1992).
Some people are considered to be defamation proof. Such folks have reputations that are so bad that it’s not possible to lower their standing in the community. What could one say about Charles Manson, for example, that would degrade his reputation?
On the other side of the coin, there are people whose reputations for spreading falsehoods are such that nothing they say is taken seriously. Since essentially nothing such persons say is believable, nothing they say exposes their targets to scorn, hatred, contempt, or ridicule. It’s likely that such people are incapable of committing defamation. The ravings of a schizophrenic street person probably aren’t defamatory.
Thus, when a noted liar publishes falsehoods that no reasonable person finds credible, it’s simply a waste of time to sue. At least for defamation.