You authorize accounts, not devices on the computer in iTunes. It's primarily to make sure your Apple ID (account) will work with iTunes, but you should authrorize any Apple ID's associated with devices you intent to connect to that computer/iTunes.
It's found under the Store menu.
On the iPad itself, if you don't want the account to be able to purchase stuff in one of the iStores, just go to the bottom of the main tab in that store (say the featured tab in the App Store), tap on the Apple ID, and sign out. This has to be done individually for each store.
Alternately, you can go To Settings > General > Restrictions and turn off the ability to install or delete apps, and turn off the iTunes and/or iBooks store; among other settings. Don't forget the passcode you set when turning on Restrictions. You'll have to restore the entire iPad to undo that mistake.