In most casinos you can opt out when making your first deposit. If that is not possible you'll have to contact their support first and ask to decline any promotion.
Casino Without Bonus
One of the misconceptions about online casinos is that you have to sign up for the welcome bonus in order to enjoy the features that are on offer. This is simply not case and quite often new players are distracted by some of the welcome bonuses that are on offer, especially if they are for large amounts.
While some welcome bonuses at online casinos do not require you to make a deposit, for example, a free spins no deposit bonus, most encourage you to engage in deposit bonuses where you can deposit up to a certain amount, such as $500 in order to receive $500 in bonus funds.
With these though, there are wagering requirements attached where you have to wager for example, x35 your bonus funds before you can make any withdrawals. Even if you do sign up to these, you are under no obligation to fulfill them.
This means that you can effectively opt out and then you can enjoy the online casino at your leisure and everything that it has to offer, such as the different slots and table games or even live casino products.
When you do this, it essentially gives you a lot more freedom of being able to enjoy the experience of an online casino at your own pace. Even if this is your preference, you can still find ongoing bonuses where the wagering requirements are not as rigid. It could mean that you can get 20 free spins per week if the online casino has seen that you have deposited a certain amount over a period of time.