So financially ruining a country isn't forcing them to change?
Also, have you seen how many products American's consume that are made in China? The government would have to organize a vote to even see if we'd all be willing to make those sacrifices for a while. Not to mention how many businesses would sue the government for losses they would take during the unrest. And why the hell would the Chinese government even let their people know American had stopped supporting them? Why not just say "Well, we're completely out of money, so you don't get any help feeding your family after that earthquake destroyed your village and farm, so sorry about that." -- Sure, people might eventually revolt, but again, the loss of lives would be staggering.
Also, the US couldn't do this unilaterally, we'd have to have a lot of support, and China has a perma-seat on the UN security council, which makes things really dicey in the realm of international politics. China could just retaliate by nixing any and all UN resolutions the US supports. Yeah, it'd be dickish of them. Then again who ever accused politicians of being sensible?
Again, I want to stress, I don't like the Chinese government anymore than you do, but the bigger picture must be considered before we simply 'make' them reform.