I had the same thing happen (see my post at alt.cellular.verizon).
1) First, I would suggest a complaint to the FCC. Per the FCC's website, the company has 45 days to respond to you. SMS.AC is a California company, so fully under the FCC's jurisdiction. SMS.AC should be fully liable for refunding all charges if you were not properly notified of pricing.
2) Second, there is information on cancelling your account on
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. If you didn't set up an account (ie someone else put your number in), there is (under the pricing section) a way to send a "STOP ALL" text to a certain phone number.For all who have had problems with this company, please file an FCC complaint. The FCC has the power to levy severe fines on deceptive marketing practices such as those of SMS.AC, but prioritizes their efforts based on complaint volume.
-MVL