Get a shovel, an axe, a chainsaw and a gardenhose. Dig a hole half a meter in diamenter around the trunk, using the shovel and the axe. Clean the big roots with the gardenhose. Cut them with chainsaw. You have to remove everything bit by bit. Very labor intensive. I would simply dig a hole half a meter in diameter and try to remove the top of the trunk, drill holes in it and cover it so that the soil on top of the trunk is level. Much less work. Will take decades to decay but who cares? Out of sight out of mind.