My cell provider is Sprint and all I had to do was text "block" then the phone number and sent it to 9999. I got an immediate message back from Sprint saying texts from this number were blocked.
I also forwarded the spam text to 7726, which spells "spam" when looking on a phone keypad. I believe most major cell providers have this option. I got a message right back asking to give them the phone number of the spammer, so I did.