You should stop giving away free bonuses and letting people cashout without documents, its very easy to create multiple accounts with VPN and get free bonus everytime and just maker wager and cashout...
I made 20 accounts with VPN (different IP) already and got free bonus on every account... People can easily cashout money without any risk on your site, so you only lost money from this promotion and no documents needed rule.
Scammers will always find a way to scam. But if you are a Bitcoin bookie/casino and you want to appeal to the Bitcoin audience, then KYC is an absolute no-go. There can be other ways of finding out multi accounts. I bet you your 20 accounts are flagged even with VPN, if guys like you are willing to waste your time just signing up and trying to reach cashout, the same guys will be finding a way around KYC.
There are other ways besides KYC too to weed out scammers.