You can refuse a casino bonus, although it may take some extra effort to make it happen.
Although a casino bonus is a very attractive and an easy way to get some extra chips to play with, there are also conditions attached to it, such as the wager or time limit. If you think this is not for you: you can refuse.
The best thing to do is to contact the customer service of your casino. But some casinos offer a possibility to cancel a bonus in the player account (fixed amount). The bonus amount is then withdrawn from your balance. Other casinos offer a select option for no bonus at the deposit screen. No bonus can be useful if you only want to play for a brief time and have your winnings withdrawn immediately.