how do you get out of a timeshare?

We were scammed into purchasing a "membership" with NH Krystal during a vacation in 2010 to Cancun. We were told we could sell our weeks to make a profit, and we would not have to pay maintanence fees unless we use the membership,which would be paid at the time of making the reservation. We paid $600 with a company to help list our weeks to sell them and were gaurenteed to sell. This has yet to happen. We are now getting bills to pay for maintanence fees, however, we have not called to book. In our contract it states these fees will be paid at the time of the reservation. We've called but have gotten no where. We want out of this membership.