Why does it say "v'Gam"?
Radak: Just like va'Yasar (he removed) all the abominations of his fathers, he did so also to his [grand]mother. This is like it explains, he demoted her from being Gevirah (refer to 15:13:2:1).
Malbim #1: Also his [grand]mother was an idolatry; people bowed to her to worship her. He also removed her Mifletzes, which pertained to worship of her.
Malbim #2: She was Kedeshah (consecrated) to the idolatry; he demoted her from being Gevirah, and also removed her Mifletzes, which pertained to her being Kedeshah for the Asherah.
What is the meaning of "va'Ysirah mi'Gevirah"?
Rashi: He demoted her from being Gevirah (esteemed).
What is "Mifletzes"?
Rashi citing Avodah Zarah 44a: It was MaFLi LeiTZanuSah (a wondrous mockery). She made the form of a male organ for it, and had 'Bi'ah' with it every day.
Radak: She called the idolatry Mifletzes, an expression of fear and trembling, like "Palatzos Bi'asasni" (Yeshayah 21:4); its fear is upon its worshippers. For the same reason it is called Eimim - "uva'Eimim Yis'holalu" (Yirmeyah 50:38).
Why did he burn it in Nachal Kidron?
Radak: He cast the ashes to Nachal Kidron [lest one benefit from them], for idolatry is Asur b'Hana'ah 1 .
Tosfos (Temurah 33b): There is a Mitzvah to burn idolatry - ["Pesilei Eloheihem Tisrefun ba'Esh",] "va'Ashereihem Tisrefun" (Devarim 7:25, 12:3). Normally, ashes of ha'Nisrafim are permitted, for the Mitzvah was done. Here is different, due to "Lo Yidbak b'Yadcha Me'umah Min ha'Cherem" (Devarim 13:18). Sho'el u'Meshiv (4, 1:53) - the ashes of idolatry and Hekdesh are forbidden, for they are never Batel. (A Nochri can be Mevatel idolatry, but a Jew cannot. - PF)
