Well, I can tell I never took Latin. Calumnize would be my verb of choice; calumniate sounds awkward and wrong to me, but looking at the etymology I can see that it came straight from the Latin.
Like Father Steve (hi, you! ), I think of calumny as a noun, though, as in, "What he said about me was a calumny". But what would be the verb (if there is one) to express the act?
Commit a calumny? Stated a calumny?