Interesting question, as marle has said, you need to know where they are coming in, but i'd like to know why, Slugs have a excellent sense of smell, so somethings attracting them, they are good climbers, and if they are not climbing, it has to be the floor, something in the tiles that perhaps tittilates their nostrils?. Anyway, be that as it may, once you've found their access point, yo can block it humanely with some copper, or be their executioner with a barrier of salt.