What to answer an author asking me if I reviewed his/her paper?
You could say something like


  "I am not in the habit of telling people whether I did or did not review their papers, sorry"


or


  "I don't feel comfortable answering this question."


Or you could defuse the question with a humorous answer, e.g.


  "I would tell you but then I'd have to kill you"


or


  "I don't remember, I always take an amnesia pill immediately after reviewing a paper"


or some similar kind of obviously nonserious smart-assery. None of these answers provide any useful information to the asker, and all of them convey some level of disapproval on your part at being asked, making it unlikely that the asker would press the case any further.