In many cases they probably had some clause in a contract that allowed them to charge you for an additional service for something. For instance, my cable company is constantly changing their rates and slowly raising the fees. They usually send a simple two line note that as of X date the service will be an additional X dollars. It's a poor business model in my opinion, but rarely is it illegal (especially if it is a reputable company to begin with).
As for your options:
My first step would have been, like you, to contact the company. In the past I've had an amazon membership (for example) that somehow got activated. It was a pain to get a real person on the line, but once I did they refunded it; all 4 months.
If they truly aren't responding to your inquiries through the proper channels, then your next option is to initiate a charge back through your payment provider. If your payment method is a credit card or check card (with a visa or mastercard logo) then the odds are very good that they will simply reverse the charges you tell them to; just politely but directly tell them that you never authorized those charges and the company has not responded to your inquiries to have them removed. If the charges go back more than a couple of months, you may have to ask for a special exception to reverse those. But generally those credit card companies work for their customers (you!) very well and will reverse anything they reasonably believe you did not intend to pay for.
Realize though, that going the charge back route will probably at best sour your relationship with the company, and at worst result in collections actions against you; especially if they did have a legal right to charge you for the service based on a user agreement.