You log in to my T-mobile , then click on profile 
Then click on privacy and notifications 
Then change what needs to be. There is no way to block msg from tmobile.
This should reduce the amounts of messages for any plan changes. notifications to just the account holder.
Unless you go to each device and if it has a messaging fitter. You block the following senders: 456,889, 1151, 5116.
Manage your T-Mobile account