AT&T doesn't provide a way to block spam text messages. They suggest signing up for a $120/year service that enables number blocking.
Yeah, mobile phone companies are pure Satan.
AT&T does provide a reporting service:
To report spam text messages to AT&T: When you receive text spam, forward the whole message to short code 'SPAM' (7726) from your wireless device. There is no charge to report mobile spam. Messages forwarded to 7726 do not count toward your data usage or voice package.
To this on the iPhone you
1) view the SMS text message conversation
2) tap EDIT at the top of the screen
3) check off the message(s) you'd like to forward
4) tap FORWARD at the bottom right of the screen
Problem is, this only forwards the message, not the sender number. When you submit AT&T replies with a request for the sender number. I had to inspect my incoming message list to find that, and I couldn't copy paste. I had to do the old memory/pencil thing.