After clicking the 'deposit' button or My Account, if it is still possible to cancel a withdrawal, a message will appear showing the words "Pending Withdrawals"
Click on the pending withdrawals link, and you will be able to see a list of the withdrawals that you may cancel.
Click the 'cancel' button next to whichever withdrawal you wish to reverse.
You will receive a confirmation message, and you will also be able to track this activity in your Account History by selecting "All"
The funds from the withdrawal that you cancelled will now be returned to your account balance and available to use.