If I'm feeling extremely generous, an app that sends me a spam message will get its notifications disabled forever. But most of the time it simply gets deleted and a 1-star review left on the store, and replaced with an app that doesn't spam me.
They are prohibited by App Store review guidelines 5.5 and 5.6:[1]
> 5.5 Apps that use Push Notifications to send unsolicited messages, or for the purpose of phishing or spamming will be rejected
> 5.6 Apps cannot use Push Notifications to send advertising, promotions, or direct marketing of any kind
Apple has responded positively to reports I've made regarding those guidelines, but it's hard to say how much of an effect it had. The DM push notifications I reported seemed to come out of the blue and were never repeated: the company could've been told by Apple to stop or they could've just been doing it as a test.
[1]: https://developer.apple.com/app-store/review/guidelines/#pus...
Seems to me that someone is holding Apple to very different standards from other companies here. If they're sending any type of spam, I'm either blind or it's so low quantity that I don't even notice.
(Aside: I periodically read your blog; great stuff.)
Thanks for the note about my blog!