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If the bonus was 125% of 500$, why did they cancel 5000$.
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Because of
their findings [#12] specified on point number 2, multi-acc, and 3, arbitration situations. It's written on their
general rules that they reserve the right to void winnings for such violations. So, not only the bonus that was voided, the winning bets were also cancelled.

Hi Holy.
The only problem is, how can we trust the casino's words?
I know the rules are the rules but who is checking if the casino that made the rules is playing by them?
If I was OP I would try to get a 3rd party mediator involved.
In case the casino denies that it pretty much shows that not everything might be as they are saying.
I'm not taking sides here, I just don't trust sites that make accusations just to not pay players. They took his deposit, they took his action. If he lost there would be no issue. It only came because he won some money. If everything is as they say they could share it with a mediation site, just OP needs to ask them to do so.
In these cases it's always statement against statement. Without seeing evidence it's hard to trust the casino. They make the accusation, OP's words here are just a result of that. So they need to show proof. OP cannot proof he doesn't own multiple accounts or what not. They are holding all the evidence.
Like I said, get a 3rd party involved, thats the easiest way.