Always notify the proper authorities or companies of suspicious behavior. The Better Business Bureau, the Attorney General, or even the local FBI may be appropriate. In other cases, consider who the suspicious person or company is claiming to be, and contact the actual company to notify them of your concerns.
For example, if you get an email claiming to be from FedEx that requests personal information, you should contact the FedEx headquarters. Let them know that this is occurring, and provide them with all of the relevant information. Not only can they help track down the perpetrator, but they will likely appreciate it.