Similar question posted by you in another thread
Find out the actual reason behind the issue whether any conflict between the employee and his/her superior. Look for change of team option if feasible.
Explain the employee that disobeying the instructions of superior @ work front is a very serious misconduct which might lead to termination of employment if proved. Explain further how termination affect will affect his /her career like BGV process etc. If the employee still adamant then follow the disciplinary process and close it. Primarily ensure that employee is given sufficient opportunity to present his/her explanation.
From India, Madras