When someone invites you to an event, there shouldn’t be an option for maybe. Treat all maybes as a no. If they show up without confirmation, make it difficult for them to find a seat or gain access to food. If they complain simply say, “I saved all the prime positions and delectable foods for people who told me yes”. The maybe is only there so that if something better comes up they can leave their options open. Eventually, the person will wonder why they don’t get invited to things anymore.