make sure the entry hole is big enough, and make the bait hard to get, eventually the crab scramble will leave the largest crabs in there.
small shore crabs can be eaten, i regularly make a bisque from shore crab and velvets, just drop them into a pan of boiling water for between 3 and 6 mins, until they just about turn red, then clean them a bit in cold water. make a stock up, then smash up the cooked crab in a bowl and add it to the stock, simmer that for 30mins then strain and use as a base for a soup or chowder, reduce it to use as a sauce for a fish or crab supper. yummy.