Why does it say "Yimalei Barzel"?
Rashi: One who wants to cut it needs an iron weapon to cut it [because it is so hard].
Targum Yonasan: One who begins to sin, the sins overpower him and cover him with an iron armor that spears cannot pierce.
Radak #1: One who needs to cut them will cover his body with armor, so they cannot harm him.
Radak #2: Yimalei is to cut off, like "b'Lo Yomo Timalei" (Iyov 15:32). One who wants to touch them must cut them off from afar with iron or a spear stick, or burn them where they grow. So Resha'im, one must seek to eradicate them himself if he can, just like he cuts thorns via a spear stick, or via others, i.e. pray that they be finished off in their place, like thorns burned in their place.
Malbim: [In the time of Mashi'ach,] people will cut their swords to make plowshares, and the thorns will be eradicated.
Why does it say "uva'Esh Saruf Yisarfu ba'Sheves"?
Rashi: The only Tikun (solution) for it is to burn it and be warmed from it. So Resha'im, their only Tikun is to burn in Gehinom.
Malbim: This is like it says in Mal'achi (3:19) - all Resha'im will be like straw, and the day to come will burn them.
Why does it say "ba'Sheves"?
Rashi, Targum Yonasan: [The Resha'im will burn when] Hashem will sit on His throne of judgment.
