Fraudulent player behavior
Common fraudulent behavior from online casinos players includes the signing up for multiple casino accounts using different identities in order to claim a bonus offer multiple times. Another form of fraudulent behavior might be the use of a graphics editing software like Adobe Photoshop to create a false winning slot machine game screenshot in an attempt to tell the casino they hit a jackpot and didn't get paid for it.
Online casinos usually lock the player accounts for these people, and it's widely believed that online casinos share fraudulent player blacklists.