The first thing Felipe Calderon needs to do is to arrest Obrador on the Mexican equivalent of inciting insurrection and put him on trial. They need to bring back the firing squad.
Yes.
It's too late for that. Obrador already has enough of a following to declare him a martyr and start marching his picture all over the country. Mexico's boat is nearing the falls sideways, and if Obrador persists, he's going to take the country to civil war. The question is, does he have the cojones to do it, and does Calderon have the cojones not to back down? There are two ways to back away from the brink - Obrador tells his people to stand down and accept the election results, or Calderon and Obrador both agree to a coalition "coercilition"? government.
If those things don't happen, Calderon will have to use force in an attempt to keep the Left down, and that will only stir more people to their side, since now people can argue that the government is getting "repressive". Want a model? Iran, 1978-79.