I'm not quite sure what your antagonist's goal is. The thread title says cripple, and I can think of lots of ways to do that. Then you say irreparably, and I can't think of one way to do that, because humans have always had some variation on an "economy," even if it was "you shoot the mastadon, I'll skin it, we'll share in the meat and I can trade whatever I can't eat for new spear points." We'll always find a way to buy/sell/trade eventually.
Then you say, destroy the economy...so do you mean in the short term? Massive disruption, panic, or do you mean America as we know it ceases to exist? Is the entire story about this, so that you'd want your Ant working on it the entire book? So it's a long-ranging set-up to the destruction?
For a short term, quick hit at the economy, I've always considered a massive computer disruption of the financial systems (banks, stock markets, companies that manage data transfer between ATM/debit kiosks and wherever they go, etc) the surest way to accomplish that. Pretty much every area of the economy is dependent on some computer system. Take down enough of them all at once et voila. If there is a way not only to disrupt, but to infect them in a way that would make it nearly impossible to get things back up and running in a matter of days (ie a true virus, one that constantly changes the DNA of the code), the interruption would be massive.
Also along those same lines, a massive interruption of the electrical grid, which would disrupt widespread systems. We had a huge power outage a few winters ago that lasted more than a week. The back up generator failed at our local grocery store and the checkers couldn't even get the cash drawers open to sell anything (even if they knew what the price of the thing was), and the one local gas station couldn't pump any gas.
Again, people would find ways around this stuff eventually, so it depends on what your story goal is.