Why did they say "veha'Nevi'im Yihyu l'Ru'ach"?
Rashi citing Targum Yonasan: Liars will be nothing. I.e. [you say that] the Nevi'im who tell us false prophecies in the name of Hashem, their words will turn to abyss.
Radak: The Nevi'im who deceive you and say that there will be Shalom, will be "like chaff in front of wind" (Tehilim 35:5). This will be their punishment.
Malbim: (They denied Nevu'ah from Hashem, which entails that Torah is not from Shamayim.) Since the punishment they prophesized in the name of Hashem will not come, it will be seen that they spoke without "Ru'ach Hashem", rather, mere Ru'ach.
What is the meaning of "veha'Dibur Ein Bahem"?
Rashi: There is no Kedushah in the words of Hashem that they tell us.
Radak: They have no Ru'ach ha'Kodesh. How do they know so?! They rely on their dreams or magic and promise the nation good, to find grace in their eyes.
Why did he say "Ko Ye'aseh Lahem"?
Rashi: The Navi says to those who say so (that the words of Hashem's Nevi'im are empty), so will be done to you (like verse 14 tells).
Malbim: The nation will punish them for scaring them with Sheker.
