No one should own any gun ever. But the real problem is handguns. They're concealable, so they're used in the overwhelming majority of crimes. The fact that he was carrying a long gun actually makes him easier to spot for police.
To answer your question more specifically, it's not really possible to ban the AR without banning semi auto rifles outright. The internal mechanics of the weapon are no different than any other semi auto weapon and the rounds used are actually smaller and less deadly than those used in deer hunting rifles (the military chose smaller bullets to make recoil manageable in full auto mode, which is not available for civilians). You can ban the features that make ARs look more like a military weapon, as New York did. But then, you just end up with this:
To get rid of the AR, or similarly deadly weapons, you'd have to ban all semiautomatic firearms, which would entail banning most of the firearms sold in America today. I'm not totally against that, but that's what you'd have to do to solve this problem.