It is important that you show some proof of the debt paid with MRS. When the collectors call you for paying on the same account, ask them to give you the debt validation. They are bound by laws to give you the latest information regarding the debt received from the original company. It will be proven out that they are collecting on that account that is already paid off. If you can gather some proof of the payment done, you can actually stop their collection activities.
I am sorry that your ex-wife charged the bills to your account. You should talk with her and make her realize that she is responsible for her debts. You are no more responsible to bear the burden of her dues.
I am also surprised how the collection agency charged $1200 on a $100 bill. There are laws in every state about the collection fees. Visit your attorney general's office and inquire about the permissible charges. The laws are the same for every collection company in your area.