How to denounce them?
Maybe you can start here:
https://www.askgamblers.com/submit-complaintAnd if I'm not mistaken, that is just one website wherein you can file your complain and they will try to be the middle man here to see if there are merits for your complain or if the casino is correct.
So you have to prepare your proof on hand and work it out with them.
aside from what btc_angela has suggested, you can also try posting your issue on casino.guru, they too help resolve issues similar(to yours) that gamblers have on online gambling casinos.