Can you fix somebody's back by hitting it really hard?

That's not a good idea, by doing that you might cause more harm. To fix someones back first you need to ask yourself why it needs to be FIXED and what is causing the problem. Back pain can be caused by a variety of sources such as the bones, intervertebral discs, facet joints, ligaments, muscles, and viscera to name a few. Not knowing the real cause of the pain will increase the probabilities of injury, while increasing the back pain or stiffness.

I recommend that if someones back needs to be "fixed" then they should seek care from licensed health care professional such as a Chiropractor, Physical Therapist, Acupuncturist, or Orthopedist. I personally recommend Chiropractic as the first choice for evaluation and possible treatment Chiropractors are well trained for the treatment of back pain or any other spine complaint.