Note that the screenshot at right is from iTunes 9.0.3. Older versions look slightly different. Parental controls are only available in iTunes 5 and later.
To access these settings, select Preferences from the iTunes menu, and then click the Parental tab. Here you've got several options.
First, you can disable access to podcasts, internet radio or the iTunes Store completely. Note that you can keep iTunes U accessible while disabling the rest of the store. Additionally, you can deny access to shared libraries. Once turned off, these items won't appear in the left sidebar of the iTunes window.