Depends what you want to do with the area afterwards - you need to get the roots out if you want to replant immediately, otherwise, chop all topgrowth down to stumps near the ground, drill into the stumps with a medium wood bit to make holes about quarter to half inch deep, fill the holes with SBK (garden centre, its a brushwood killer), cover and leave alone. Don't spill it on the soil, wear gloves.