Malbim: Sanctify yourselves, and purify the Bayis from Tum'as Avodah Zarah, i.e. the images that Achaz carved into it, and bury the Kelim [that Achaz defiled].

'Rashi': It is idolatry [Radak ? that Achaz put by the Mizbe'ach]. It is more despised than a Mes, like Tum'as Nidah; blood of Nidah is despised and putrid. Malbim ? idolatry is Metamei one who moves it, like a Nidah.