I did get your email, but unfortunately it got buried under a whole mountain of them. Sorry about that. This is indeed the best place to post support questions, as it also helps other people who are having trouble.
What you found was a battle of the priorities, of sorts. WP works with hooks comprised of filters and actions. To decide who receives what, I use a filter to load/remove recipients.
The option ‘Do not notify me of global forum messages’ is in fact provided by the BbpNNS Opt-Out plugin, which had a priority of 10. The instant notification option was provided by the Digests plugin, which has a priority of 1000.
So the Opt Out removed the users and then the instant notification reloaded the ‘forcers’ back in.
Right now I’m upping the Opt Out priority to
PHP_INT_MAX so that it’s the ultimate rule into who gets removed from the recipient queue. Note, however, that core subscribers will always be added back to the notification. After all, it’s Global notifications that we block.
I’ll release the fix to bbpNNS Opt-Out shortly.