Rashi: When I will punish them [they will perish].

Radak: This refers to the punishment of Bavel. They will perish, like it says above (verse 11). Madai u'Faras destroyed Bavel. Also the gods lost their [reputation for] divinity and worshippers - "Kara Bel Kores Nevo Hayu Atzabeihem la'Chayah vela'Behemah..." (Yeshayah 46:1) They were broken and carried on animals, i.e. the gold and silver on them (was taken away).

Radak (16, citing the Kuzari): When one investigates them, they are nothing, like a forgery. If you probe it, you find trickery. If you investigate the Divine, it is found to be like gold.

Malbim: When they will perish, it will be revealed that they are Hevel (nothing). Our verse contrasts idols to Hashem's attributes in verse 10. "Hashem Elokim Emes" - "Ki Sheker Nisko." "Elokim Chayim" - "v'Lo Ru'ach Bam." "Melech Olam" - "Hevel Hemah." "Mi'Kitzpo Tir'ash ha'Aretz" - "b'Es Pekudasam Yovedu."