You don't say what type of nutsedge you're trying to control. If it's yellow
nutsedge, controlling it is about all that can be done until it runs out of
energy and stops sending up shoots. Purple nutsedge, on the other hand,
might have some method of killing it, from what I've seen in some obscure
article on the internet; but don't hold your breath. Spray the shoots with
anything that will kill the shoots, but don't expect to kill the seed-nut
because it will continue sending out shoots until it depletes its energy
source. Of course, you can alway try to dig it out, but it no guarantee that
you'll find the nut. Just keep spraying, and pull out the occasional lone
shoot. The nuts are edible, in case you wish to gather a fistful and clean
them and roast them. Just about the time you begin to enjoy them, they'll
probably die off!

By the way, nutsedges are allelopathic (like walnut trees) and secrete a
chemical that discourages growth of other plants in order to assure its own
survival. Definitely a good idea to get rid of it if you can.