I had no idea this existed, but my wife recently started receiving spam via SMS on her phone. To report the spam, you can just forward the message to the number 7726 (SPAM), and then the service responds to ask for the number it came from.

Apple also has a support article on how to report spam sent via their iMessage service. You’re supposed to send a screenshot of the spam to imessage.spam@icloud.com and include the full email address or phone number, and the date and time you received the message.