Once you have initiated a withdrawal, the transaction is being automatically created and sent to the blockchain. Please allow some time for it to get confirmed by miners. We are not able to affect the speed of this process as it depends on current network load.
If you happen to change your mind to withdraw, you may cancel the transaction while it is not sent to blockchain yet (i.e. does not have a transaction hash).
This can be done on your Account page, "Latest transactions" section, or on the Transaction History (Reports) page by pushing "Cancel" button on the right from the withdrawal status.
In case you do not see this option, it means that the transaction has been already sent to the blockchain and is currently being processed.