I suppose that could be done, but it would have to be linked to an existing transaction, within 120 days of the transaction date, and the refund would have to be equal to or less than the amount charged. That would be a little damaging, but chances are that anyone who managed to gain access would have no transactions of their own on your site, therefore again no motivation to do this. You don't have unlinked credit turned on, do you?
Practically speaking, anyone who gains access to your hosting can do much worse things to you than just use your Authorize.net account. They can, for instance, modify your ordering process so credit card details are collected on your site and mirrored to their site. Security starts and ends with your hosting, worrying about the Authorize.net login ID and transaction key is kind of trying to close the barn door after the cows have already escaped.