There is point about multi accounting in the contract (it is obviously forbidden).
But there is no clear information about the procedure of investigation, i.e. they accuse me of multi accounting (again sorry for repeating, without any proof), I claim that this is my first and only account and ready to provide any documents to support my claim - if both parties don't change their view, the question goes beyond the contract, somewhere in the field of common law.
https://sportsbet.io/about/terms-and-conditions17. Cancellations, Suspensions and Closure
17.1. Without restricting our ability to rely on other remedies that may be available to us, we may suspend and/or terminate your account, cancel any outstanding bets and/or confiscate any or all funds in your account at our absolute discretion if:
[...]
(vi) we determine that you are breaching any term of these Terms and Conditions;
[...]
17.2. You acknowledge that Sportsbet.io shall be the final decision-maker as to whether you have violated rules, terms or conditions in a manner that results in suspension or permanent barring from participation in our Websites.
Seems quite clear to me. Is it fair? Maybe not. Is it lawful? I don't know - talk to a lawyer. But I don't think you have a case for a type 3 flag here on this forum.