Here's the problem with a simple "LOL SHUT IT DOWN" button:
They didn't (And potentially still don't) know the integrity of the shit down there.
The surrounding seafloor itself might be weak. It could be fractured - that's something that's slowing down the relief wells - they have to check for that shit. Simply turning off their pipes without knowing the extent of the damage could have resulted in just a massive fucking crater where all of it used to be, with oil coming out at a much more rapid place.
The blow out preventer might be fucked. That's the big risk with the top kill - If it isn't strong enough to withstand the pressure, it goes, and again, same problem.
There is MASSIVE pressure there now. Before, the sea floor was enough to hold it. Now it's got holes in it, potentially is fractured, and is just in general nowhere near as much integrity as it did before they started drilling.
Just flipping a switch could have made matters much worse without knowing the details.