A substance you can use to stop the root from regrowing if you can't remove. Drill holes in the stump, wearing rubber gloves pour the liquid into the holes and cover with something so nothing else can get at the liquid.

a bin bag and some string should do the job to cover it .

All info given is correct - SBK is a liquid brushwood killer - the bottle says to dilute, but I don't dilute it when used in the way Moon Grower describes. Don't spill it anywhere else, it'll sit in soil for months. Obviously, this advice assumes you've just got a stump left, having removed all the upper trunk and growth.