First, cut the nipple off flush (or thereabouts) with the fitting. Then, make two cuts near each other inside the nipple, going just deep enough that the threads start to show. At that point you should be able to chisel out the little "pie wedge" you cut. After that fold in the remaining chunk by chiseling near the edge of the cut until it bends in enough that you can use that big old screwdriver you have to continue the folding it in. At some point you will be able to grab it with pliers, twist, and pull it out.