If you changed your mind about playing with a bonus, head over to your profile's bonuses section and you will be able to find an information box on the active bonus, click on "cancel" and you're all set!
Please keep in mind:
When a bonus is canceled, any winnings earned during the use of the bonus will be removed from the account. Winnings become cashable after the wagering requirement is completed.
Depending on the amount of progress of the bonus, currently available funds may be partially or fully removed from your wallet.
Once a bonus has been canceled it cannot be undone.
For more details, please read the Bonus Terms and Conditions.