The phrase "to clamp down" means to take strict measures or actions in order to control, suppress or prevent something from happening. It can refer to a variety of situations where authorities or individuals are taking firm action to curb an activity or behavior that is considered undesirable or dangerous. For example, governments may clamp down on protests by imposing restrictions and using force to disperse the crowds; parents may clamp down on their children's internet usage in order to prevent them from accessing inappropriate content; a company may clamp down on its employees' expenses in an effort to reduce costs. The phrase suggests that measures are being taken to enforce rules and maintain control, often with the aim of restoring order or preventing further problems.