You can, but you don't need to, because a cashback bonus never obliges you to do anything. It's kept in its own balance, separate from your Real Funds, and it has no effect on your trading or on your withdrawals: there's no trading requirement attached to your own money, and you can withdraw your Real Funds exactly as you could before, whether or not a cashback bonus is on your account. If you'd rather not use it, simply leave it. It expires 30 days after it was credited and is removed from your account on its own.
If you'd still prefer not to have the welcome cashback bonus, let us know through the chat and we'll take care of it. See Bonus and cashback conditions.
Related: Bonus and cashback conditions · Can I withdraw my cashback bonus? · Why can't I trade with my cashback bonus? · How do Zentrader bonuses or cashbacks work?