Is the money you are giving them a monthly allowance, or explicitly for food and other school materials?
The reason why I ask is that the terminology can be important.
Where I grew up, "allowance" meant "your parents give you money that you can spend as you want".
If that is what you intend, then you can't say "no, you can't buy this with your money". As bobobobo mentioned, your perspective on what is important will differ from your kids', and setting (from their perspective) arbitrary rules on what they can and cannot spend their money on will be confusing and frustrating to them.
If you truly want it to be a generic allowance, then you should focus on educating them on budgeting, possibly by offering to help them plan a budget for how they spend their money, and assisting by reminding them of their planned budget when it looks like they may be straying; just remember, though: it is their budget, and they should have final approval on it (and they should also have to deal fully with the consequences should they overspend their budget, or plan poorly).
On the other hand, if you really just want to give them money explicitly for lunch and school supplies, then you need to make that clear. Furthermore, since you know that they are already violating that rule, it would be appropriate to put some new restrictions in place. Namely, I'd suggest figuring out how much of the money you give them that you expect to go to school supplies, and deduct that from what you give them. Moving forward, if they need school supplies, they need to ask you, or go with you to buy them, and you get veto power on anything that you don't think they need (i.e. the plain 3-ring binder instead of the more expensive binder that features Gundam units on the cover).
For lunch, you can either give them enough to cover food, with the understanding that if they use the money on other stuff they don't eat, or you can start packing lunches.
However, there will likely be resentment that they can no longer buy the stuff they want. You can either discuss possibilities for making money around the house (I agree with the blog Torben cited, and don't believe household chores should be rewarded with money, but by agreeing to take on chores that aren't normally their responsibility, perhaps a financial arrangement might be appropriate), or let them deal with the restrictions for a suitable period of time (a month or more, depending on how well they adjust, or how much they complain) and then consider giving them some money for them to use at their discretion.
Giving them money to use at their discretion creates a lot more opportunities to learn about budgeting and responsible spending.