Individual devices can be unenrolled from the Apple Beta Software Program at any time. To remove an iPhone from the program a non-beta version of iOS must be installed on the device. Click here for instructions on how to downgrade from iOS public beta.
Once the iPhone is unenrolled, the Apple ID used to register for the Apple Beta Software Program can be removed from the program entirely. To leave the program, go to the unenroll page here, and click Sign In at the bottom of the page under "How do I leave the program?"
After the Apple ID is deregistered from the program, Apple will stop sending information related to the Apple Beta Software Program via email.