You could've asked the person to elaborate or be specific, or even contradicted or questioned them- without saying "you lack <important skill>" and "try to <be useful> rather than <being unimportant>".
Why so mean? Was the meanness intentional, or...? Genuinely curious.
"[F]inance/accounting/banking" is not one coherent blob of thieves or parasites or whatever labels otherwise apparently reasonable people get away with attaching to anything in the field. Of course it's not a divinely perfect field either (not that anyone is claiming that), of course it has big and important problems. But especially if you work in or near finance, it gets really annoying just how accepted and endorsed rampant prejudice is in the rest of society.
One helpful technique is to read what you've written out loud and consider whether it's something you'd say to someone's face.
Really, grow a thicker skin.
Here were my thoughts:
1- We could have had an interesting discussion about finance
2- instead, it went "money wonks"-> "you lack critical skills" + "standard claptrap" -> "you lack some important read-a-newspaper-in-the-last-seven-years skills".
3- Back-and-forth snide, snarky attacks are far more boring (to me) than actually discussing points of contention, different points of view, so on.
4- I definitely appreciate that you took the trouble to link to the Emergency Economic Stablization Act, but it feels to me like the conversation had already soured before that.
5- I'm writing what I'm writing not so much because of this particular instance, but because mean comments in general tend to derail otherwise interesting or could've-been-interesting threads. Which feels wasteful to me. A mean comment has a souring effect that's IMHO not worth the short-term entertainment value.