I have heard the city of Chicago is using baking soda blasting to clan graffiti from brick work with little damage to the brick work. I would image this would be a specialized field and would most likely have to be hired out but better then having to change the bricks.:confused:
Myself I have made brickwork look much better by using various color stains matching the brick and touching up the obvious paint drips with paints that blend in well with the brick. It can almost go undetected if done well.