After day 1 of year 7 DS had spent twice as much as the agreed amount for his lunch. I can't afford that and he words with DS and DS decided he wanted packed lunch anyway (unrelated to his telling off).
Anyway DS said he has no friends on packed lunch and every was on school dinners so he ended up sitting with year 8s who never spoke to him etc so I reluctantly agree he could go back on school dinners but keep spending to a minimum like take drink and snack and buy a main meal and dessert.
This seemed to be going well until yesterday. When he bought toast, drink, pizza, main meal, dessert and two more drinks.
Shall I send him with a jam butty or tell him he's had his quota? 
In all seriousness though I can't afford this so what's the solution? There doesn't seem to be an option of setting a limit on the account.