Handling Unwilling Workers: How to Approach Issuing a Warning Letter?

In my company, for the last 15 days, there has been no work for laborers due to the lack of assignments. Today, I called in one person for work, but he is not willing to do the assigned tasks. Consequently, I will have to issue a warning letter. Can anyone provide assistance with this matter?

Thank you.

Warning Letter

Dear XYZ,

It has been brought to the management's attention that you have neglected the job assigned to you by refusing the instructions of your superiors. This is to warn you that such negligence and misconduct will not be acceptable to the management in the future, and serious action, up to and including termination of your services with us, will be taken.

This serves as a final warning.

Please take this matter seriously and ensure that you fulfill your job responsibilities diligently moving forward.

Thank you.

Sincerely,
[Your Position]