To whom is this Pasuk referring
Rashi (on Pasuk 20): It incorporates someone who quashes his prophecy, someone who transgresses the command of a Navi or who transgresses his own prophecy. 1
See Sifsei Chachamim.
What does "Anochi Efrosh me'Imo" mean?
Rashi (on Pasuk 20): It means that he is Chayav Misah bi'Yedei Shamayim.