Someone "testing" stolen? credit cards

Hello,

For the past few days we have had an account (now two) where a person is making $2-$10 gifts through our TNEW contribution page.  Each of these transactions are on a different credit card but all with the same name.  We think he/she is testing to see if the cards are valid.  It doesn't appear to be a bot because there is like a 5-10 minute break between transactions.  We require the CVV on all online transactions so it appears they have that information as well.  When we identified the first account we disabled the login but now we have a second account with the same name and similar circumstances.  We reported the suspicious activity to our credit card processor (Vantiv) but haven't heard anything back yet.  Has anyone else had this issue?  What did you do to stop them?  We are awaiting instructions on whether or not to refund the orders in case the legit owner issues a charge-back.

Thanks

Michael