App store still showing beta release of El Capitan months after withdrawal from beta programs?
This worked for me! After unchecking the beta software enrollment in the system settings, I ran:

sudo softwareupdate --clear-catalog

man softwareupdate doesn't document this flag, but it seems to clear pending updates from the App Store, and on the next run of the App Store it will repopulate based on the current configuration of your system, therefore it will not show the beta update, but it will still show other regular updates for your system. 

man page: https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man8/softwareupdate.8.html