You can forward the SPAM text message to 7726 (it spells "spam" on a phone's keypad).
AT&T, Sprint, T-Mobile and Verizon all accept spam reports through this number. You may receive a follow-up message after reporting a message, asking for more information or to confirm the number the original message was sent from.
https://www.consumer.ftc.gov/articles/how-recognize-and-report-spam-text-messages
