I am having the same issue, I set my machine to no longer receive beta updates following the final release of 10.12.1, and just now the next public beta popped up in App Store. I went to the public beta website, enrolled my machine again, restarted, and immediately un-enrolled once again, yet the update remains.
There seems to be a bug in Sierra that does not clear the beta from the list of update sources. You can try clicking on ‘Check Now’ in the App Store settings in System Preferences or clear the catalogue with this Terminal command: