As the others have said you need to be clear and firm and need to rewrite your contract.

If you need a more gentle way to bring it up in hopes of not having them leave before you want them to then I would write up a notice when they have used up their 2 weeks. A simple letter just reminding them that as per the contract they signed they have used 2 weeks of vacation and any additional time off with be paid.

This will inform them now that what they used was their vacation time and will let them decide if they want to do summer camps at their own expense.

The contract should be very, very clear and should note in writing any and all possibilities so they can't make a loop hole. So instead of just saying vacation I listed examples (a day with grandma, a day with mom, a day off for an appointment, on and on).