set the soften brush to a much smaller size than you need, 1 mostly is still to big , and hardness to 100%, helps keep it inside it's borders a bit, but not that much, too (set it back for textures)
one other thing that sometimes helps though: after you just made one short softening click in landscape and then press ctrl-z, you'll notice how your one click mostly gets undone in not one, but several steps. this can be used to some degree.