The scammer can easily respond to any questions from the victim with "Sir, if you don't cooperate, we can't help you." They may even be able to convince the victim that not cooperating will result in liability for the fraudulent charges. Any defense against a scam really depends on the victim being knowledgeable enough to know exactly when the scammer is lying, which pretty much entails being more knowledgeable about today's financial systems than the scammer is, which is unlikely due to the sheer complexity involved. The scammer has all day to study these things, it's essentially their job, and they may have even been previously employed as the real officials they're impersonating. It's unreasonable to expect the average person to have this level of expertise. Really the only defense against such scams is to avoid these systems entirely, (they're insecure by nature), which again for most people is more trouble than it's worth.