FULLERTON – Debate over a billboard that has inflamed emotions on both sides of the immigration issue on Monday reached into Fullerton, where protesters masked part of the controversial sign there with a U.S. flag.The billboard for KRCA/62's Spanish-language news program reads "Los Angeles, CA" - with "CA" crossed out and replaced with "Mexico."The flag protest heightens antagonism about the long-simmering issue, which was spotlighted after Gov. Arnold Schwarzenegger lauded "Minuteman" volunteers last week for patrolling the U.S.-Mexico border.