You cannot limit the amount a student spends. Students first pick food then they go to checkout lines. If the system blocked transactions because of daily or weekly limits, the cashier would have no choice but to throw the food away. In addition, consider that cashiers deal with thousands of students each day so this has to work really efficiently. They cannot pause to explain to students “sorry hun, you parent blocked you today”. It would delay every other student in the same line.
We recommend having a conversation with your son or daughter and not fund the lunch account beyond certain limit you feel comfortable with. If she or he runs out of money, they will need to take food from home. After few times students learn how to budget the lunch allowance. Many parents in your shoes have been able to execute this strategy successfully.