In order to close your account you need to ensure you have no open positions and that you have withdrawn all of your funds.
To see how to close a position click here.
To see how to withdraw your funds click here.
Once your account has a $0 balance, please email support.au@capital.com from your registered email address requesting the closure of your account. Any feedback you can provide on the reason for your account closure would be much appreciated and used to improve our products and services in the future.