I have a Nokia Lumia and have has the same problem for a month and a half now! Did you ever get it to stop? I have contacted TMobile over 10 times now and haven't gotten anybody who can figure it out. We get at least one text a day asking us to sign up on three different phones. The kicker is that I am already signed up for it!
Log in to your account at my.t-mobile.com and look for the Profile: Change Communication Preferences setting. You can deselect the option to be notified by mobile, e-mail, etc. There were complaints about this a month or two ago about "notifications" that were ads sent by the My Account app.
Small consolation for the annoyance, but these messages shouldn't count against your message plan allowance.
Message was edited by: chromejob