TidBITS:

AT&T offers a way to report unwanted SMS/MMS messages, but I had to ask on Twitter for the instructions, as the steps are not obvious to those of us who don’t use SMS/MMS regularly. The free AT&T Mark the Spot app for reporting bad cell coverage or calls also includes a button for reporting SMS spam that explains, with too little detail, how to do it.Here are the steps for AT&T; I’ll add other carriers as I receive information, as AT&T is the only carrier I have an account with.

As everyone points out, this is an increasingly annoying issue that infuriates customers.