You can easily tell that the deposit is suspicious and you can easily locked the account or ask user to KYC may I ask how are you going to to spot those kind of activity?
All the casinos more or less have this practice. Maybe they are right or maybe they are using this as their main weapon to hold withdrawal. No one detects it before deposit.
No casino even reveals how they detect because if they reveals, there will be people who can abuse/by pass the system. I have to agree that this is ridiculous but there's no alternative even. So far I have seen lots of reputed sites to hold users fund and saying they had abused the system but not clearing why they abused.