The way I found to lower an embedded pipe below grade is to first cut the pipe as low as possible. This involves 2 cuts, the first is to remove enough so that when the second cut is made the remaining section can be handled safely. Use a reciprocating saw with a long enough metal cutting blade.
After the second cut the pipe should be 3/4-1 inch protruding from the ground. Next make 2 cuts so that the pipe is cut in quarters. With a cold chisel and short handled sledge pound each of the 4 quartered pipe sections inwards to the pipe.
Hammer each section at a time until they touch the inside of the pipe. When done the pipe will be below ground level and ready to be filled with asphalt.
Alternately, once the pipe has been cut so that it is close to the ground cut the inside wall with a new saw-zall metal blade and again at 30 degrees from the first. You may not cut all the way through, but the resulting 2 slices will be enough to snap the pipe with pliers. It can then be twisted and rolled from the ground.