Can Rebel Missions such as The Final Battle be destroyed by the Dark side player with card effects to keep the Rebels from gaining their benefits -for example can you use Superlaser Blast to destroy The Final Battle?

According to the rules insert in the Endor cycle, a mission is completed when it enters a player's victory pile. So while Superlaser Blast does allow you to destroy an objective you control, it would still enter the Light Side player's victory pile, meaning the mission is completed. So no, you can't use Superlaser Blast to prevent the Reaction of The Final Battle.