A well-known garden pest, Slugs will eat vegetation and are only occasionally found indoors, in damp areas such as cellars. Despite their appearance, Slugs are harmless and is only a minor agricultural/amenity pest.
Control, ultimately, is best achieved by curing the damp problem. Removal of outdoor vegetation and debris close to the affected area may also help. Slug pellets may be applied, with care, in accordance with the product label.
Use a trained professional pest controller.