I suggest you start by politely asking the passenger behind you to stop banging the back of your seat. I know you don't expect to accomplish anything that way, but it's important for form's sake. If the other passenger keeps at it or become verbally abusive, call an FA or visit them at their station. Be very polite and matter-of-fact with the FA. You're depending on him/her to resolve the situation in your favor, and if you resort to hyperbole or name-calling that's less likely to happen.