Q: Who earns a living driving their customers away?
A: A taxi driver!

Explanation:  To “drive someone away” means to repel them, to force them to go away from you.  This phrase is used to describe a battle (6th paragraph) in a war when it is said, for example, “The attacking army was driven away by the defending army.”

A taxi driver, in a different sense, drives their customers away from where they are to where they want to be.  This joke plays with the phrase “to drive away,” something that most businesses do not want to do.