I feel sure that if anyone threatened Clinton face-to-face publicly with those words they would have been charged. Now that you mention it it seems there was at least one incident where someone was detained and possibly arrested for saying far less than what this lady said.
Actually, my question was SHOULD they have been prosecuted. I'm guessing you would say no...
It depends. If they threaten a President or Head of State I think they should be arrested, prosecuted and have their day in court.