As owners of a liquor store, should we discourage sales to alcoholics known to us?

My family runs a liquor store in our own neighbourhood, so most of the customers are known to us.

My family runs a liquor store in our own neighbourhood, so most of the customers are known to us. Occasionally, we get calls, and visits, from family members of people with a history of alcohol abuse. They plead with us not to sell alcohol to their loved ones. What should we do?