playing devil’s-advocate a little here, but why not just leave it.

Nature will break it down eventually – sure it will take decades. Some animals will die, others may be better off – that’s life. As a punishment for humans we should cordon off the hole area from human activity and leave it as a nature reserve for a hundred years. If we can’t be trusted to manage the environment then we shouldn’t be allowed to use it.

Note I think everyone needs to take collective responsibility not just BP. Sure they take the brunt of the costs, but it affects us all in the long run. We wanted cheap oil/gas and so the safeguards were only as good as people and governments were prepared to pay & legislate for.