My chief revenge fantasy is this. To find out what number and service is sending you the spam, go to the message options menu and check the SMS message details. The details include the mobile phone number of the message sender and the message centre number of the cellular service provider. Get the originating number.
I would set up my laptop as a server. In this blog you would find python code on how to send SMS from a server. I would then go to a public place that offered free internet. I would put the number into the code, and the message would be "Stop sending spam stupid, and yo mama is fat!" I would then put the send code in an endless loop, and go and order a latte and sip it slowly while my laptop churned away, eating their message credits up.
Of course I would never do this, because I need to have impeccable ethics to work for some of the people that I do. That is why this is a revenge fantasy only.
In the meantime, the next internet billionaire, is the guy who figures out how to stop spam at the source.