A good contract which clearly spells out the limits of their licence AND some <meta> tags in the web-pages' header which clearly identifies these pages as being made by your system and perhaps even identify the individual licence this page is linked to. GOOGLE will pick up the <meta>-tag and then checking if your customers are honest becomes as easy as GOOGLING for your custom <meta>-tag. Of course it can easily be circumvented but that would need some "hacking" into your code and if your customers are that good, there is probably little you can do to stop them from defrauding you.