Well, theres a formal policy in place with the merchat provider.

Call them, and ask them what their process is for disputing a chargeback.

The more paperwork you have, the better.

IP Address signed up from
Any email correspondance with the customer (proving they are using your services and bought them willingly)
Copy of your TOS and cancellation policy
Anything with proof of a money back gurantee is nice as well
etc...

I must stress however, if the purchase was fraud, you have NO recource. the merchant provider will withdrawl the funds from your account and more then likely charge you a penalty.

Hope this helps...