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:

imessage.spam@icloud.com

1. Include a screen­shot of the message you have received.
2. Include the full email address or phone number you received the unwanted message from.
3. Include the date and time that you received the message.