Round Up and 2/4/D are excellent choices. Each of them have to be applied to leaves. Round Up will kill all plants and grasses, 2/4/D just kills broad leaf plants. This is commonly sold as Weed-B-Gone. Neither of these will sterilize the soil, so replanting can be done almost immediately. Allow some time for either product to work and apply a second application after a week or so if there is still signs of live. The chemicals are absorbed by the leaves and taken into the roots so it does take some time for the full effect to take place, but you will see some leaf curling and wilting in a few hours. Avoid letting spray mist drift on to other plants. Neither product is particularly toxic to humans or animals, but don't make a cocktail out of them.