Once the leaves have black spot, nothing will make it go away, but you can prevent it from spreading.
The basics are, first and foremost remove all leaves with spots. Pick up any on the ground as well, and put them in a bag, into the trash, don't compost. This reduces the amount of spores flying around in your garden.
Second, only water your roses early in the morning so that the leaves dry off quickly. If you can water them with a drip or soaker system that doesn't wet the leaves, that is best, but in general try not to wet the leaves and especially not to wet them in the evening so they stay wet through the night. Open invitation to fungal infection.
Third, prune or space out your rose bushes so that they have good air circulation between them. This again helps keep the leaves dry and also reduces the fungal pathogen from being passed around.
You can then use a fungicide, or anti-fungal treatment. Some people spray with a baking soda/water solution or diluted milk to prevent fungus. It changes the pH of the leaf surface so it's not hospitable to the spores. You have to keep doing this at least weekly, all summer to keep the fungus at bay, but it's the most non-toxic method. There are dozens of chemical fungicides you can use also. Be sure to follow directions religiously, and wear protective clothing, a hat, glasses, and I wear a painter's mask so as not to inhale the stuff.
