A support document by Apple (last modified today, July 30) that I’ve never seen before details how users can report iMessage spam (unwanted messages) directly to Apple (via Beau Giles):
If you’re seeing unwanted iMessages (spam) in Messages app, you can report those to Apple.
To report unwanted iMessage messages to Apple, please send an email with the following details to: firstname.lastname@example.org
I’ve never been the target of iMessage spam, but it’s good to know that Apple has a basic reporting tool in place. Some users experienced iMessage “denial of service” spam messages earlier this year; in the support document, Apple doesn’t explain how they will act against reported spam.