I am sure that they can make or have a high tension steel plat that can take that kind of forces. The answer to stop that leak is at that joint just below the open end. The are just making harder than what it should be.
Usually they would totally remove the malfunctioning valve by unbolting it and the bolting a new valve (that is open) on, then close the valve.
This would work if the casing bowl and the malfunctioning BOP`s were still in position.
I have not seen any pictures of the BOP stack which leads me to believe that the casing has actually moved up out of the borehole some.
(if this is the case then they can not kill the hole by restricting the flow coming out of the casing)
If this is the case then the only way of killing it is to enter the bore hole either by reentering the original borehole or by directional drilling into the borehole downhole