Try selling a Ferrari for $5000 and see what happens.
Everybody knows that a Ferrari can't cost $5000, but no fucking body knows what the price of some stocks should be. It's just too much data from a gazillion sources, and it's extremely chaotic. A company could see a huge 'proper' increase in their stock value due to some local war in Africa or the untimely death of a pop star. Now we have programs specialized in finding those trends and profitable stock-and purchasing it automatically, and we have programs specialized in deceiving them.
I think those kind of programs should be both forbidden by law. Automatic buyers are extremely dangerous, and the only way to decrease the risk they pose is by putting a human being in the chain of command, with the responsibility of giving approval before placing the orders. The difficult part is that it should be a human being with some common sense.
Having the bad guys placing their orders manually would cripple to a great extent their ability to manipulate markets. Of course one of these guys could make a program simulating human behavior, but then they wouldn't be able to place thousands of orders per hour.
My 0,02 €