Create a hollow in the middle that will collect and hold water. Then toss in some black topsoil. This will cause it to rot within a year or two. Then bust it out with an ax or maul. I don't think anything short of a tractor and chain can get rid of it now. You can, however, lop it off nearly level with the ground with a chain saw - this is what I had to do on the ATV trail I made.