Corkscrew willow can be a rampant grower. Fortunately, it withstands rather severe pruning. In Europe they are often pollarded---cut all the way back to the main trunk each winter or at least every couple of years. You need not prune that severely, but I just wanted to assure you that you can prune the tree almost as much as you'd like to keep it in shape. Avoid pruning during rainy periods, however, or late in the growing season.