If you use iTunes, you may never use the iTunes Store. It’s baked into the app, with a button at the top of the iTunes window, in the navigation bar below the iTunes LCD.
But you can hide the iTunes Store if you want. Go to iTunes > Preferences, and then click Parental. In the Parental Controls preference, you can choose to hide certain aspects of iTunes.
Check iTunes Store, and then click OK, and you won’t see the iTunes Store button any more, and you won’t accidentally click it.
Interestingly, this also hides iTunes Radio. So if you want to turn off that feature, you can do so, but you can’t just turn off iTunes Radio, and not the iTunes Store.