Welcome to Android Central! Which phone do you have? Are you certain it's the Auto Backup photos that is taking up space? Typically, if the Auto Backup folder is syncing with your Gallery, it's going to be primarily thumbnails, unless you download the photos back onto your phone. Have you deleted the original photos on your phone already?
You can stop syncing your Google+/Picasa folders with the stock Gallery app by going to Settings>Accounts-Google>[your Google account], and unchecking either Picasa, Gallery, or Google Photos (not Google+ Photos). Then go to Settings>Apps>All or the App Manager, select Gallery, and Clear Cache/Clear Data, then Force Stop. The next time you open Gallery, the database will rebuild, this time without any of your Google+/Picasa photos.