When you block a transaction, that means that someone somewhere has the information of the cardholder. By having that, they may then have access to their bank account and therefore start digging for more than just the money—personal information that can be used for identity theft and so on. Your action may block a transaction, but part of the damage is done already, and we don't yet have any control over what kind of information has been stolen at this point.
On April 8th, 2019. See this statement in context.