Once you confirm the withdrawal via email there is no way for it to be canceled. Always make sure that you are sending your coins to the correct address and you should always do a small test transaction first to see if the coins will arrive where they are supposed to before sending the full amount.
There are two ways to cancel a withdrawal before it's been confirmed via email.
1. If you click on the "Cancel transaction" button in the email, the transaction will be canceled and the coins will be returned to your balance right away.
2. If you open the pending transaction in the app, there is a "Cancel transaction" button there too.