Turningg On / Off In App Purchaces

No problem, go into the build tool and hit the "!" symbol in the top right corner. There you will get your options. Turn Confirm Donut Spend to "ON". I think that's what you're looking for. As far as I know there is no way to prevent in app purchases outright. at least inside Tapped Out. There might be parental controls within iOS that allows this.

** UPDATE - (assuming you are on iOS) go into Setting>General>Restrictions>Enable Restrictions>Turn Off/ON In App Purchases