I had the same thing happen this month from a Premium TXT Messaging provider called Mobeebuzz.  One day I simply received a text message out of the blue saying "congratulations you are now subscribed to Mobeebuzz."  I absolutely did not sign up for the service and they have not responded to my requests to unsubscribe my number.

When I called Verizon to dispute the charge, they indicated that there's nothing they can do from their end.  All I received was an explanation of what Premium TXT Messaging services are, which isn't the point.

I checked their website and unfortunately Mobeebuzz appears to be located in Cyprus, so it's unlikely that anyone in the US can do anything about them.

I understand if Verizon can't tell whether initialization of the third party service was legitimate or not, but I expected a bit more assistance in getting the charges credited and at least blocking future subscriptions to Premium TXT Messaging services.  I have never used such a service and don't intend to and wouldn't care in the least if they were completely disabled for my account!!!