Certified GKH Gardening Expert
There are many ways to go about this. Sprays will ensure that you kill the spores, which is how it spreads. Ground soaks can kill what is in the ground, but will not touch what is in, and on the plant unless there is something fto let the plants take it up. This can be an oil (coconut has several fats and oils that help plants take up nutrients and chemicals) or synthetic chemicals.
While there are many treatments, such as a ground application of DOLOMITIC LIME and WETTABLE SULFUR (This combo is wonderful, and what I use personally) or fungicides that contain copper (This should be a last resort thing) it may also be necessary to kill the spores of the visible "rust" on the leaves.
This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will give you information on the care of Hollyhocks: https://www.gardeningknowhow.com/ornamental/flowers/hollyhock/tips-on-hollyhocks-growing-hollyhocks-successfully.htm