Fitness Connection Seeking money after legal cancellation in writing. Sparks Nevada

Cancelled membership for myself and two of my children in person (in Sparks, NV) and in writing to Corporate office in Texas.  Called to confirm both cancellations. Was treated extremely rudely by Location Manager in Sparks. Never used gym again after cancellation. Monthly membership is $30.00 for each, but I receive bills for 149.00 for each of us totalling 450.00!  I cancelled on January 31, 2013. As I understand it, bills include a yearly fee that would have come out at the end of February to cover the upcoming year. Why would I be billed for a year when I have already cancelled membership? Bills also include two months dues after I gave termination notice, and additional fees I am unaware of. 

Turned me over to a collection agency who now calls me every day (even on Saturday and Sunday) at 8:00 am in morning. The collection agency is billing under my children's names and I assume trying to add it to their credit report when they are minors and legally unable to enter into contracts. I intend to notify the Attorney General's office and Better Business Bureau. Stay clear of this gym. I have previously had memberships at six other gyms in five states and never had this problem. They will not get a dime from me.