Inject expandable polyurathane foam under the wellhead base.
It may expand inside the pipe and block the flow.
A really big roll of Duct Tape.
The problem is they’re having to go into the side of the BOP with the top some what open and the bottom pushing tons of fluid. They have to seal the top section before they can pump back down hole otherwise the cement will follow the path of least resistance. What they need is a 12 inch frac ball, to bad nobody makes one.