If the update checker has already detected a beta, and then beta detection is turned off, the notification in the status bar and (if turned on) system notifications will still show at the moment, due to the detected update being cached.
As a workaround, after turning off checking for beta updates:
Go to /dopusglobaldata\Global Data and edit globaldata.omd in a text editor.
Remove all of the attributes that start with update_, i.e.
update_download_low="65536"
update_download_high="786446"
update_size="57743976"
update_lang="english"
update_new_low="65536"
update_new_high="786446"
(The values in quotes may not be the same. The names are the important part.)
Then fully exiting Opus (via File / Exit Directory Opus) and restart it.
Edit: The workaround shouldn't be needed anymore from 12.15 onward.