How do you get rid of Bacterial Leaf Spot in lettuce?

You don't. Once the pathogen is established in a lettuce crop, there is little that can be done to salvage the crop. The commonly-used copper-based bactericides recommended for other vegetable crops can be phytotoxic on lettuce, and deliver only about 50% efficacy in eliminating the disease. Cultural controls such as the use of certified pathogen-free seed and non-sprinkler irrigation systems (the "sprinkler-splash" spreads the disease around from plant to plant) are the most commonly used methods of controlling the disease.