Use the element of suprise. I had a high school teacher tell a story about when he was in the military and was on leave and was at a local bar when some guy wouldn't leave him alone. The barender fnally told him he should take the guy outaide and he did. now I was in about 9th grade and a small guy myself (graduated at 120lbs) and I could look down at this teacher. He went outside and told the guy he'd give him to the count of three to shut up. Then counted to One and knocked him out. He skipped two and three.