The way Apple's restrictions work, you can't build global opt outs like that. Services, for the record, would love to be able to do it; in fact, I was once begged by some companies in the analytics space (think Flurry) to build a global opt out for their products on jailbroken devices, and after contacting other companies I found out they all wanted to do it but couldn't (so I built it).

That doesn't stop the app from prompting for the in-app purchases, it just prevents unwanted transactions.

Go download Talking Tom (it's "free") and see what parents complain about. Turning off purchasing doesn't stop my 2 year old from hitting one of three or four "upgrade" buttons available on screen at any given moment, bringing up a popup or opening a browser or the app store for cross promotions. I would have actually bought the damn thing because of the entertainment it provides if it weren't for refusing to support the total sleaziness of that monetization method.