The spores which cause the leaf spot are your enemy and it sounds like the area where all the affected plants reside is chockful of these spores. This means they are in the leaves, in the stems, in the mulch and so forth. Anything that will make life difficult for the spores will help and based on your comment, a fungicide program may be also required.

I would read the information on the link below and then apply these fungicides as described. I would also replace the mulch with new mulch. When spraying, remember to spray the plant leaves' top and bottom as well as the stems.

As long as you spray the affected and unaffected plants, cutting the plant down to the ground is not going to do much because the plant will grow anew and get infected from leftover spores in the environment.

Consider reviewing just why is the problem so bad. Is the humidity always high in the area of the garden? Or are you maybe watering the soil too much? Etc? High humidity levels are just perfect for the development of the spores so any steps that will reduce the humidity will minimize the problem.

As soon as the plants have gone dormant, throw away all the dried out bloomage and leaves in the trash. Then pick up any plant debris that you find and also throw it in the trash.

Luis

Here is a link that might be useful: Diseases of hydrangeas