Ans:
This subject tends to get into muddy waters as the whistle-blower laws and other legalities surrounding it make the right course of action difficult to determine. Whistle-blower laws protect workers who report the illegal activity of their employers as long as they do so with good intentions.
Even if the accusation was unfounded, you could still get in trouble for terminating one of these employees if the complaint was filed in good faith. That being said, if an employee reports the company knowing the accusation is false, you are free to terminate them.