A weedkiller containing Glyphosate, such as Roundup, will kill weeds down to the roots through the foliage. If you dont have too much, then you can paint it on with a paint brush. If you use a spray, then as Darcydark says, you must effectively mask other plants. You can get a hood that fits of the nozzle of a spayer, or make one yourself with a funnel.
landscaper2 has the answer, and I`ve bought a large funnel which I place over the weed and spray down it with Roundup. Obviously the funnel must be in close contact with the soil.
selective weedkillers do exist ie kill broad leafed plants on lawns, plants around trees and weeds around rose bushes but don't know what other kinds are out there.