Step 1:

Suspend there account, they may be trying to use the server to do something bad. Customers who signup with frad credit cards are always up to no good.

Step 2:

Unless you have there signature on file there is nothing you can do about a chargeback. This is one reason why I call all my new customers.

I got a chargeback for the same reason about 6 months ago but it was only like $15 so not bad but not sure how bad in your case.