If you have created a withdrawal request but HAVE NOT confirmed the withdrawal on your email; you may cancel the withdrawal request via 2 methods:

1) Through the Activity tab. Select the Withdrawal currency and select the Withdrawal Status tab. Click the Cancel button to cancel your withdrawal.

2) You can do the same through the Market tab in the section below the graph. 

If you have already confirmed the withdrawal in your email, the withdrawal can no longer be cancelled.