Short answer - you can use "fiddlesticks" (a bit old-fashioned as others have said though not archaic) but not in this way. It is simple an interjection, used by itself.

Agreed. Fiddlesticks is archaic. On top of that, it would typically be used as an interjection to disapprove of something. Grandmother: "Would you like another piece of chocolate?" Grandson: "I don't think mom would let me" Grandmother: "Fiddlesticks! She's too strict. Have the chocolate"

Not really. First of all, 'fiddlesticks' is a very antiquated way of saying something IS nonsense. Whenever I hear it, personally I think it's a much older person, or else a younger person saying it to be funny, imitating the language of an older person. It's the same as 'hogwash' or 'horse feathers'. It IS nonsense. Not someone doing something that is nonsense. (Noun vs Verb.) In English, if you want to tell someone to "stop your nonsense" in other ways, you can say: "Cut the crap." - can be substituted for the profane word too. :) This is a bit more harsh, and should only really be used in places maybe outside of work or formal settings. This is often said between friends, both seriously and in fun. "Quit/Stop fooling around." A safe way to say nonsense, usually can be said to kids. "Quit/Stop horsing around." Another safe way to say it.