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.