Under the rule 5.6.
If we determine, in our sole discretion, that you are using the Double Spend methodology, Sportsbet.io shall void all bets and winnings. Specifically, if you win, then confirm your deposit on the Blockchain and attempt to withdraw, all winnings will be confiscated and your account will be closed permanently. We shall also exercise this right where similar activities are attempted from any connected accounts.
Study double spend.
A double spend is an attack where the given set of coins is spent in more than one transaction.
My guess is that they are suspecting that you spent (deposited) the same coin twice to trick their system. You might want to give them the deposit tx IDs to prove that your deposit amount was legit (if it is). But looks like you will have hard time here. This betting sites are always there to take your money and we all know it.