You can cancel a bonus yourself from the Promotions page. What you get back depends on whether you have started playing with it.
Before you have placed a bonus bet
If you have not wagered anything from the bonus yet, cancelling returns your qualifying deposit to your cash balance and removes the bonus funds.
After you have started playing
Once you have wagered from the bonus balance, cancelling forfeits the bonus. The remaining bonus balance is removed and the deposit is not refunded, because your deposit became part of the bonus balance when the bonus activated.
If you only want your money out, it is usually better to finish the wagering first.
When the cancel button will not work
A game round paid for by the bonus is still open. Finish the round and try again.
You have sports bets placed with the bonus that have not settled yet. Wait for them to settle.
Bonus balance and withdrawals
Your bonus balance is separate from your cash balance and cannot be withdrawn until the wagering requirement is complete. Only your cash balance can be withdrawn.