It's rather saddening and painful to see other users suffering from gambling addictions. Although I'd like to sympathize with you, I personally see it as bad communication from your side. As @JollyGood already mentioned, you asked them to close the account from which you were contacting them; not both accounts registered under your name. Despite that, I understand your frustration because they have your data available at any time and could have seen that you have two accounts; however, it doesn't necessarily mean that this data is accessible at any time by any employee in customer support. I doubt that they're going to go through your details to see if you have multiple accounts or if their system is portraying that when you reach out to their support. They do what they're told, as you didn't specify to have both accounts closed. Had you specified it, both accounts would now be closed, and there's a chance you wouldn't have gone through all this.

I believe that there are applications or extensions that block gambling websites, acting similarly to parental controls, perhaps you should look into that in the future.

Best of luck.