As laid out in the forum guidelines, for all account related inquires you should call *611 from your celluar or 1-866-558-2273 from a landline.
You can request a data block if your concerned with data usage. Sending SMS or receiving SMS does not result in additional data billing. If your seeing additional data it usually means something is set on your phone to use data, typically push notifications/email/weather/stock updates, these are the more common things that use data these days because data is used to deliver you updated information or messages.
MMS should not be charging you for data used, but bear in mind that when you have data enabled the phone may be doing a lot of stuff in the background, checking for email, checking for updates etc. I personally recommend having data turned off even at the expense of not having MMS available for that reason if you're using a smart phone without a data addon/plan.
Can't say for sure if that's what's going on, just a guess. I'd call tech support to get to the root of it.