If you rent the entire plot of land out to the retailer, I would have thought it is their problem, not yours.
However, if you rent individual plots I see your problem.
I would put up some signs stating that it is private property and unauthorised users will be clamped. If that doesnt work, you might want to consider employing a clamping agency for the short term in order to discourage the nuisance parkers.
You *could* issue parking tickets, but probably people will ignore them.
Beware though, this area is an absolute minefield and you will need to ensure you have proper signage etc.