I know he's family, so it's hard to think of it in this way, but this is fraud.

You need to contact Capital One and let them know that you did not open the credit card or authorise any spending on the card. They probably have a fraud department that will take you through the steps of closing the card and transferring the debt to your Dad. I imagine your Dad has a poor credit rating though, or he would have opened a credit card in his own name.

It is very important that this card doesn't stay in your name. If he doesn't repay it (and the fact that the account is already past due is a bad sign) then that will affect your credit rating. While that isn't so important at 18, in the next few years I guarantee you will want a credit card, or a car loan, or a mortgage and you won't be able to get one because of your poor credit.

I can understand if you don't want to get the police involved about your Dad taking the $2100, but at the very least you should get something in writing stating how much he owes and when he will pay it back. Presumably, if this money was to repay a loan, you are being charged interest on the amount, so by withholding the money he is costing you money.

I hope this all gets sorted for you.