If you've cut it down, you need a drill with a medium to large wood bit and a bottle of SBK, brushwood killer - drill into the stump (not just in the middle if its a big one, nearer the edges) to make wells about quarter to half inch deep, fill up the wells with SBK and cover with something. It'll kill it, but won't make it rot down any quicker.

You can have a pro grind into wood chips. It will be gone in just a few minutes.