Since there are trillions of dollars worth of software patents out there, most owned by US companies, this doesn't seem like a good way to proceed even if it were legal. You might be able to stop future applications for software patents. But really there's no bright line between software and hardware anymore -- you really need to reform the entire patent system.
I agree their economic value is probably negative, but because of the broken patent laws companies currently need them. Since companies like Google and Apple have recently spent billions of dollars to buy relatively small collections of patents, the entire existing pool would probably cost a few trillion to buy. It's really not a good idea to make laws that cause companies to spend this much money, and then declare their value to be zero. It's also probably not legal.
