Rushes are a sympton of wet ground, they will only come back. Best solution would be to sort the problem, put in some drainage. Sorry I can't help you on the herbicide.
You can buy or maybe rent a thing called a licker that kills the rushes with herbicide.
It is pulled behind a tractor and the herbicide is delivered too high to hurt the grass but is apparently effective on the rushes.
Is your rush problem in a lawn or field. yes it is a sign that you have wet ground and seems like draining your garden is your long term solution.Rush licking is only successful in agricultural fields. Otherwise spot spray with round up if you haven t many rushes but there are herbicides that will kill rushes and not kill grass. if rush problem is v bad it may be an idea to spray off everything and reseed.