A The law regarding private parking spaces is fraught with difficulties, so you should seek expert advice before taking any further steps. But as general guidance, if you own the land and have clear signage explaining that the parking spaces are only for the use of customers, anyone parking there is entering into a contract with you to purchase something from your shop. Not doing so is a breach of that contract.
Drivers could get around this by buying a 20p packet of mints, which leaves you with the dilemma of enforcing a minimum spend. But this could be extremely off-putting for customers: many people leave shops empty-handed having had a good browse, and even though you probably wouldn't fine such people, the threat that you technically could hardly makes them want to come into your shop.
If a driver did persistently abuse your free space, you could invoice them for a penalty fee. But the driver does not have to provide you with any information about their name or address, and you might have to battle with the DVLA to get this information based on their registration plate. Also note that you can only claim a fee from the driver of the car, not the owner, and if the owner denied being the driver you might have to resort to the messy business of CCTV evidence.
Some private parking companies will issue "fines" of up to £100 for unauthorised parking, but as a private individual you actually have no right to do this. You may issue an invoice to cover damages, but the amount you claim is extremely tricky to justify legally. You might claim that the driver deprived you of the chance to sell a £100 case of wine to another customer who wanted the space. But how do you prove that?
If you genuinely feel that you're losing business to parking cheats, it may be easier to engage the services of a specialist private parking company to help you draw up a fair system. But be aware that private parking schemes can sometimes create as many problems as they solve.