Rose and other flower gardeners routinely deadhead their plants when blooms are spent. This is simply the process of cutting the flower from the stem after it has faded and died. Most plants benefit from this procedure. It prevents them from producing seeds, thereby conserving energy for continued bloom production. But knockout roses don't produce seeds, so they'll continue extended blooming periods without any help. But if the presence of petal-less dead roses bothers you, it's fine to deadhead your knockouts to tidy up their appearance.
Hold the spent flower in one hand and clip the stem with shears in the other. Always use clean, sharp pruning tools. Cut the stem back right above the first leaflet with 5 leaves.
Prune out any deadwood, twiggy growth or damaged stems that you notice while you're there with your shears.
Deadhead enthusiastically following heavy rains, which will dislodge many loose petals from spent knockout blooms.
Drop healthy spent rose material into your garden or onto your compost heap.