Q: Can we require an employee to receive psychological counseling or treatment if their behavior has become a hindrance to their job performance?
A: You cannot require employees to receive any medical treatment, psychological or otherwise, as a condition of continued employment. But you are not without recourse. Even if an employee is protected by the ADA (i.e., he or she has a mental condition that rises to the level of a "disability"), that employee is still subject to discipline, up to termination, if he or she violates your policies regarding misconduct.
FINAL TIP: Remember the "golden rules" of employee discipline: evenhanded enforcement and careful documentation.