Always fire a warning shot. There are two main camps on the subject of warning shots: in the air and into the ground. I demur. The best warning shot is center of mass. It conserves ammo and definitely gets the message across with little chance of misunderstanding. (Center of mass warning shots also minimize collateral damage.)
One should always continue offering proper warning shots until an aggressor’s off switch is fully engaged, although, if the aggressor’s off switch is not located near his center of mass, a polite tap-to-the-head warning shot might be required. Oh, be ultra polite and make it two.