First things first, go to your iOS device's Home screen page (hit the circular button on the front of the gizmo) and find the 'Settings' app. It's the one with three silver cogs on its icon.
If you can't find it, use Spotlight (by swiping across the Home screen from left to right) to search for it.
Once you're inside the 'Settings' app, choose 'General' from the list of options.
Scroll down and tap on 'Restrictions'.
As you can see, this page lets you put the kibosh on loads of features, from receiving FaceTime video calls to downloading 18-rated movies on iTunes.
First off, though, click 'Enable Restrictions'.
Next, you need to set a four-digit passcode. This locks out nosy kids who are trying to revert back to the old settings. Pop in a passcode (twice) to continue.
Those previously locked options are now available for you to tinker with. Now, there are two ways to go about solving your in-app purchase woes.
The problem that most parents face is that once they put in their password (say, to download a new game), it kickstarts a 15-minute grace period during which you can buy whatever you want without having to re-insert your credentials. This is when unfortunate shopping sprees occur.
To avoid this pitfall, choose 'Require Password' and then set it to 'Immediately'. Now, you'll need your Apple ID password for every single purchase, either on the store or inside an app.
If that's not enough for you, simply set 'In-App Purchases' to 'off' and you'll put a stop to all IAPs forever.
See? We went to download some Mighty Falcon tokens in Angry Birds Star Wars, but our restrictive iPhone wouldn't let us buy them.
With this restriction in place, you won't be able to download Smurfberries, Bux, Donuts, Diamonds, Gold, or Gems ever again!
Well, unless you turn in-app purchases back on again, of course.