Clematis wilt is a fungal disease that may have come with your new plants. The disease is worse in years when the weather adds additional stress like our current dry spell and hot temps. You were correct to clean up diseased plant materials. Spraying the ground wont help as the disease spores need to splash from an infected plant to new clean material to infect a new plant. The large flowering species are more susceptible, so if you have a site that you need to replace go with the smaller flowering species. There are some growers who say the plants may come back from the root even after a year. I haven't worked with infected plants so I don't know this for sure. Good cultural practices are very important to the health of clematis. Well drained soil, good sunlight, no debris around roots, don't cultivate near the roots, good irrigation practices, and clean up any diseased material quickly when it happens. No other sprays or fumigants for the soil are recommended.