I believe one of the solutions as a protection mechanism against welcome bonuses, or any kind of deposit bonuses is, you don't want to hear it, but it's a solution. It's KYC. Offer generous bonuses for players, especially instant bonuses without KYC or another form of protection mechanism from cheating, then it's expected that there will always be users who will multi-account in the site until all the budget for the bonuses are drained.
Good way for passing KYC protect from cheater and bonuses abuse but I don't think all agree with KYC, not all gambler want to KYC and give document ID to gambling platform actually they want identity publish. Maybe have to stop with bonuses for new sign up member for protecting cheater, why have looking hard way for passing KYC with Bitlucy, but waiting with update have informed by OP and hope everything well and all member can get back withdrawing money.