Drill several large holes (or make several cuts with a circular saw) in the upper surface of the stump. Fill the holes with neat Roundup. If the stump is exposed to rain, cover the top with plastic and tie it round the stump with a piece of string (this is to prevent rain washing the Roundup out). Do this every 3 months. I had a tree cut down in May 2012, and I have applied Roundup once every 3 months since then. The stump is now a dead white colour, so I am assuming that the roots are also dead.