Just crimp it.
I have been screaming the same thing since it started. Most people are unaware that to place something over a pipe that is flowing requires that the flow be allowed to continue (think of how hard it is to screw on a nozzle on a flowing garden hose). A simple fix would be to lower in an valve / fitting assembly with the valve wide open. After attaching to the pipe simply close the valve.
I am not familar with the blow out assembly device they install but I would think that something along the same line could be inserted over the pipe and engaged. Slip a box over the pipe while leaving some length of pipe sticking out the end to install the above mentioned solution, seal the openings of the box and then engage some sort of mechanism to crimp or seal the pipe inside the box. It may not completely stop the flow but would reduce it dramatically...
But then I am just a simple Civil Engineer, and don’t work 5,000 feet below the surface where nothing is simple, and I am sure that some one working for BP or adivising them has already thought this up...