Why not just lower a large fitted boulder over the top ? That would certainly be heavy enough to overcome the pressure.

There are, as I understand it, concerns about pressure inside the well itself, that if the pressure is totally cut off at the top, the well casing, which keeps the oil from pushing out into the surrounding geology, could burst, making the ”leak” far worse and harder still to contain.

We have a running discussion on ways to close the well and related information: http://www.casavaria.com/hotspring/groups/building-the-green-economy/forum/topic/new-ideas-for-how-to-cap-runaway-oil-well/