My iPhone 6s and Apple Watch 1st gen were both enrolled in the developer betas. I removed the beta profile from both devices. The watch is now on OS 3.0 (14S326), and yet after multiple restarts of both devices, I'm still receiving watchOS betas (currently watchOS 3.1 Beta 2).
How do I force the watch to stop receiving beta OS updates and move back to the stable channel?
I ended up unpairing the watch (Reset -> Erase Apple Watch Content and Settings), repaired the watch, and I'm no longer receiving beta OS versions through software update.