Yes, you can cancel an active bonus. In order to do this, click on "My promotions" in your Account Overview . To cancel a bonus, simply click the "Cancel" button for the respective bonus. Please bear in mind that you will forfeit your remaining bonus funds as well as any resulting winnings from bets placed with bonus funds by doing so. Once cancelled, the bonus cannot be redeemed again or credited back to the account.
However, any other funds on your account, displayed as "Withdrawable funds", resulting from your own (real) funds are not affected by the cancellation.