They've already made their own rule for the countries that they're dealing with and it's in favor of them. So all of the people that boycotts them or not, they won't care.
You decide if all casinos based on Russia will be boycotted by you or you don't do anything as that doesn't concern you and they're just businesses that are residing there and as well as affected by the war.
sanctions will be effective if the country that is subject to sanctions, has weaknesses (depending on other countries) while so far the boycott and sanctions imposed on Russia are not effective at all because they know that foreign countries need their gas and oil.
And Russia, it doesn't seem to show any weakness despite all of the sanctions that has been placed upon them. Whether there's a huge crowd boycotting the chosen online casinos.
It won't be much of effect I guess as it's just a part of the whole that has been still standing despite all of those challenges they face.