From which hairs should he take?
Rashi: From the few [that he wrapped in his Kenaf]. Radak - they represent those who were exiled to Bavel.
Malbim: This hints to many that Yishmael ben Nesanyah killed.
Why should he burn the hairs?
Rashi: This is a Siman for Achav ben Kolayah and Tzidkiyah ben Ma'asiyah; Melech Bavel will burn them.
Radak: The Resha'im among them perished in Bavel, also Achav and Tzidkiyah ben Ma'asiyah.
Why does it say "Mimenu Setzei Esh"?
Rashi: From these hints that I hint to you, punishments will go out to all of Beis Yisrael.
Radak: From the fire that I commanded you to burn, fire will go out to Beis Yisrael, both those in Yerushalayim and those in Golah. It is a hint to eradication.
Malbim: Via this (Yishmael killed many) people went to Egypt, and no remnant remained from them, like it says in Yirmeyah (Perek 42).