[jlgreco]

Exactly, "Fuck off" gets the full message across loud and clear. It is also short enough that you can deliver the full message without any risk of interruption; just two syllables that can slip under the radar after you have the subjects attention.

  [looking the person in the eyes]
  "Hey [whoever]"
  [repeat as necessary until you have eye contact, at most one or two more times]
  "Fuck off."

There is no chance for them to interrupt you mid-protest to make excuses, and it sends a clear signal that the subject is not up for discussion.