How to stop iOS "update from beta" notifications?

Having been bothered by similar notifications, I found that the best way to suppress these was to simply turn off Automatic Updates - which are typically enabled by default.

1. Go to Settings
2. Go to iTunes & App Stores
3. Toggle Automatic Downloads to Off.

Background
I would suspect any Beta updates to be no good fit for long-term use as they are often a little buggy and intended for temporary use/evaluation only. Then there is the whole security side of things (which I won't get into now). If you are after a little more reading, try this article which explains how to suppress updates in greater detail - I wouldn't be surprised if beta versions aren't handled a little more forcefully than the regular iOS releases.

Resetting your iOS device may be the best option yet. If it is applicable to your situation, go for it.

Let me know how you get on, what worked, what didn't? I'd be interested to follow up with you on this.

Why don't you just back up to iTunes to have the local back up copy of the iPhone and restore using it. This way you won't have to manually upload your photos or save any application data. With iTunes it's pretty straightforward and safe.