Blox-o

From BoardGameGeek:

This variant of Nine Men's Morris adds "blocking" positions to the standard board. Each time a player gets three-in-a-row, he removes a piece of his opponent, but also adds a blocking pin to the line with his three-in-a-row. This block then prevents said player from making another three-in-a-row along the same line. This additional element removes some of the simplest winning patterns from the standard game. The gameboard (from 1923) includes extra spaces for the blocking pins.