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.
- Go to
Settings - Go to
iTunes & App Stores - Toggle
Automatic DownloadstoOff.
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.
Let me know how you get on, what worked, what didn't? I'd be interested to follow up with you on this.