I did too, and so far I've been able to end the messages with "STOP"
They all appear to comply with that older school SMS norm. Who knew?
If you do it for a few, they tend to end overall. Now, instead of getting a few a day (right?!?), I get one out of the blue.
I did not "STOP" Bernie. I like Bernie. I don't like how it went with Bernie and all the people trying for better. Those messages have been considerate, infrequent, etc...
So far, all of the others hit pretty hard and fast, but do "STOP"
This suggests some entities, like Bernie for example, do maintain a list but are not necessarily sharing it. Other entities either share their lists, or will send messages for hire.
Your mileage may vary. Hope sharing these experiences helps.