Multi account is truly forbidden in almost all of the casino but in any case that their claims is wrong and you didn't have any alt account on their site then the best solution is to talk their support and ask politely with your current issues.
But if you truly do it on purpose then you should not react about that since that will mostly happen when a casino/casino sportsbook do when they found that theirs a illegal activity happening in their site.
If OP didn't actually do it then it's possible that this was their scheme but a lot of this happened before and it just proves that they really commit to make another account that's why they got banned or their account was closed. If talking to support politely will help to solve the problem then he shouldn't be here now complaining/reporting the scam accusation on the said casino site.