Now that you have parental restrictions turned on, you can easily disable in-app purchases. You may need to scroll down the screen a little bit to located in-app purchases in the Allowed Content section. Simply slide the On button to the Off setting and in-app purchases will be disabled.
Many of the restrictions offered in this section function implicitly, which means disabling app purchasing removes the app store completely and turning off the ability to delete apps removes the little X button usually displayed when you hold your finger down on an app. However, apps that offer in-app purchases will still do so if you turn off in-app purchasing. Any attempt to purchase something within an app will be met with a dialog box that informs the user that these purchases have been disabled.
If you are disabling in-app purchases because you have a small child in the household, there are several other useful settings, including the ability to restrict apps based on the parental rating of the app.