Yes, we can and do. Just like you can buy a gun and shoot it at the range or hunt with it, but you can't shoot your ex-wife no matter how much she deserves it. You can also buy a car, but it's not okay to run over your neighbor with it. If a ten year old wants to stab a classmate, a #2 pencil in the gut would do as much or more damage than a steak knife. We cannot ban every tool that could be used to cause harm.