You guys should be able to cancel those orders. Call, identify yourself, and tell the guy to "Straight cancel order number XXXXXX, selling XXXXX shares of MF at market." It's an unusual order, because market orders are usually filled before they can be canceled, but since the order has not been filled, it should be cancellable. If you get any static about it, ask to speak with the person's supervisor. Sooner or later, they'll do what you ask, because if they don't, you can make real trouble for them, in that you can make them pay you for any losses you suffer from their refusal.
I'm speaking as a former stock and commodity broker. Don't "change" the order, just cancel it and replace it later.
And yes, if you don't get those orders canceled, you'll get the worst possible fills. You'll be raped by the market makers.
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