Ummmm that's technically impossible once you build muscle muscle tissue deteriation is physically impossible because newly formed ligaments were created plus your gemetics and everyday physical activities contribute to your calf size as well and the only way to really loose muscle is if you didn't use that particular muscle as often and seeing as how you want to loose muscle in calves that's basically unrealistic however you can do simple excerises that can tone them and make them appear more slim
Don't use them, you'll lose muscle. But know you might end up losing muscle all over your body if you are sedentary. Your balance will also be affected as the stabilizing muscles around your calves will also lose strength.
The only thing you can do to actually lose muscle is to let the muscles atrophy, which means stop using them for anything and basically just wither away. Anything you do that's active at all will either maintain the current level of muscle or increase it even more based on what it is.