How would you improve an email from a supervisor?

You received the following message via email from your supervisor. After reviewing the message, identify one change from the list below that you would make and thoroughly explain why you would make that change. 

1. Word Choice
2. Tone
3. Subject Line

Subject Line: Stop Working Overtime NOW!

Message: 

Hey,

When I reviewed this week's timesheets I saw that way too many people worked overtime. This is costing our company too much money and needs to immediately stop. Going forward, before you work overtime, you must get approval from your supervisor. 

Thanks,

Supervisor John

Remember to answer each part of the question with a minimum of one paragraph that is 5-7 sentences with limited grammatical errors.