Simply state your position and leave it at that. And never get angry. If there is a workable solution, identify it and try to get agreement on what be done by whom. Ask the person to stop complaining. Explain that he or she will be happier by working more and complaining less.
Troublemakers are the misfits, the malcontents, the inciters, the meddlers, the hooligans.
They aren't just victims of an unfortunate set of personality traits. They mean to cause harm. Get them professional psychological or psychiatric assistance. Among the most common indicators are slovenly appearance, chronic absenteeism and tardiness, dishonesty, frequent hangovers, temper tantrums, insubordination, extreme aggressiveness and disruptive relationships with others. Other than demonstrating interest and concern, recognizing the need for professional assistance in diagnosing and treating the problem, offering help and referring employees to specialized sources of information and assistance, there is very little else that most businesses can do for emotionally troubled employees.