If you don't want to pay for SMS spam, then don't pay it!
Make a note of each illegitimate SMS (or the legitimate ones and subtract) and deduct the total cost from the bill. Include a letter to the effect that you are disputing the bill for this reason, and you want them to block SMS spam.
You may want to cc your state regulators as well.
I don't know about T-Mobile's policy, but Verizon is very aggressive about protecting their customers from SMS spam. AT&T Wireless was also that way, since they didn't charge for incoming SMS; I don't know what their policy is now that it's Cingular.
Mark --
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does not emerge from voting, party politics, or public debate. Si vis pacem, para bellum.