Keeping slugs out of a square foot bed is no easy task! Over the years I have tried many methods but found none to be really effective under all conditions. Using bran can help (buy from fishing tackle shops) but is not a 100% cure. In fact there is no simple organic solution and the only way to control slugs is to use a whole battery of defences! (Sorry bad joke!)
I used this method on a square foot bed a few years back. The bed was surrounded by two copper wires spaced about 10mm (3/8″) apart. They ran in a loop round the top of the timber edging. Each wire was then connected to a terminal of a 12 volt battery via a 1 amp fuse (this is VERY important). This created a low voltage electric fence.
I watched one evening as a large slug slowly crawled up the timber. It got to the top of the board and made contact with the first wire. It arched over that and then settled on both wires, when it made contact it fell off. Smaller slugs get caught across the wire and are killed immediately. They need to be removed.