A simple approach is for the sponsor to require dealer active presence during specified dates and times. Violation means forfeiture of a security deposit, or public display of those who left w/o approval, or no future invitation. Once several promoters enforce similar conditions, dealers will stay and at minimum occupy the space.
Using fill-in is another useful way to use vacated space; however, if the original dealer has a contract and made payment for 3 days, and leaves after 2 days, reselling the same space will require an additional contract provision including how to determine that a table renter has actually forfeited some of their time.
Personally, I disagree with Mark's dealer-is-dominant approach. IF - IF - a bourse is intended for collectors, then the rules must benefit collector attendance, regardless of any dealer inconvenience. A clear contract, with clear statements of purpose and appropriate enforcement should solve the problem. ON THE OTHER HAND - if a bourse is intended for dealers' benefit, then the logic changes, and no one should complain if one, some, or all of the dealers vanish in the dark of night. (See 1984 treatise of Robert Irsay on skipping town under cover of darkness.)