I fell victim to the same problem. I have Boost service.
I tried to text stop, quit & end, none of those worked. I called Flycell c/s right after i realized what happened. I told them to stop my service, they told me they did just then. I then ask for the them to send me a stop text, they told me my number was not in the system.
I thanked them & hung up & called Boost c/s. I told them my issue, they told me they would block anything from 69999 & I would not receive any charges.
We'll see if it works, hope it helps someone.