If granted the applicable administration right (Admin - Remove Card Accounts), you will be able to delete an unmapped account. This will remove the account from the system and all transactions on it. It will also stop any future transactions being loaded on this account so it is an option you need to be particularly careful with.
To delete an account:
The Account Search window appears.
The Account Search screen appears.
If you have the applicable administration right and the account is unmapped, you will see a icon to the right of the account details.