Spam is a serious issue for some T-Mobile customers (UPDATE: T-Mobile blocks the source)
The source of the junk messages is a 4-digit shortcut that begins with 9***. But the number changes with each message which means that blocking the number is of no use. And since the spams aren't being sent in response to anything you've signed up for, you can't send a "STOP" message to get them to stop. Some of the spam contains messages of a sexual nature, so if you have young children using your T-Mobile account, you should make sure that they are not receiving these messages.
T-Mobile is reportedly working on a fix. If the problem with spam is particularly bad on your handset, you might want to consider a change of phone numbers. If you get one of these messages and it contains a phone number, you should forward the message to T-Mobile at 7726.
source: Reddit via TmoNews
UPDATE: T-Mobile has informed us this morning that it has blocked the source of the spam. In an email, the company told us "Our teams have now identified and blocked the sources of the unwanted text messages. We apologize to any customers who may have been inconvenienced."