If you haven’t already done so, you should make it clear that lateness is a disciplinary offence. If you cannot resolve the problem informally, you should then follow your disciplinary procedure — holding disciplinary meetings, issuing warnings and ultimately dismissing the employee.
Bear in mind that an employee who has worked for you for more than two years may claim unfair dismissal if you do not follow your disciplinary procedure. It is unlikely to be fair to suddenly dismiss an employee on the basis of previous episodes of lateness if you did not take formal action at the time.