How can I delete backed up Google+ photos off of the phone? I know Flikr has support for this, and I don't think there is a way to do this natively with Google+, so is there a 3rd party app or utility?
Using the Photos app, tap the hamburger menu (the three bars in the upper left corner), scroll down and choose
On Device. You should be able to delete these without touching the backup. If you are prompted
Delete Everywhere, select cancel, you are in the wrong place.
You could also use a file explorer app, such as ES File Explorer. Your photos are stored in DCIM>Camera. You should be able to select all and delete.
But if you are asking for something that will auto-delete them as you go, there doesn't seem to be a way to do this with Google Photos. Since they have recently been integrated with Google Drive, a new solution may arise soon.