The fake reviews will not have a chance to trick or slander the casino because the casino will try to defend and proves that they are not guilty. Even they will try to investigate it to find which people want to get them down.
The reputations can go away with one mistake and everything that they build from a long time ago can ruin. That is why the casino will not trying to make a mistake and they see a problem, and they will help their customer to solve the problem.
You're absolutely right about it mate, building a good reputation for a gambling business really takes years and if they managed to earn a good reputation they will certainly do anything to protect it even fake reviews will not justify the a gambling business once they have a good reputation standing. For sure once it has an anomaly that would going to drag down the name of the institution every representatives of the gambling business would really going to find out the solution to the problem and would not let their gamblers or players feel the problem too.
A single negative news or review won't ruin a good reputation of a gambling site, because many of its supporters would react about it depending that the rumors are not true and the gambling site would prove it that they're clean, simple as that.
A mere accusation without any solid proof will never be accepted as a fact to crumble down a certain business, it takes a lot of legal process before coming to a conclusion whether they are guilty or not. Long running online casinos would never scam or do anything foul play to ruin their reputation, in my opinion.