Any overtime worked by non-exempt employees must be compensated, regardless of whether the overtime was authorized.
If an employee has worked overtime hours that you didn’t authorize, you can start by reminding them about about work expectations outside of their scheduled workday, making it clear that the company does not expect or permit employees to work unauthorized time and that working without permission is subject to disciplinary action.
Beyond that reminder, and assuming you have a policy that requires overtime be authorized ahead of time, you can (and should) discipline employees for working unauthorized overtime, just as you would for any other violation of your policy. An oral or written warning may suffice for a first offense. Repeated offenses may warrant further corrective action, up to and including termination.