You can do this right from the poker client.

1. Log in to your poker account
2. Click My Account at the top right of the client.
3. Then click on ‘Withdrawal’
4. Then ‘Account’
5. Then ‘History’
6. Click on the ‘Pending Withdrawal’
7. Finally, click ‘Cancel’.

Please remember that if your payout status is listed as ‘processing,’ it won’t be available to cancel.  Only withdrawals with a status listed as ‘pending’ can be canceled.