1. Go to start > run > type: gpedit.msc and press OK button.
2. Computer Configuration > windows settings > security settings > Local policies > User Rights management
3. Look for Change the system time on right panel.
4. Remove the users & groups who should not be allowed to change the time.
5. If necessary, add the user who is allowed to change the time. (workstation users should have limited user accounts and should not be allowed to change system settings anyway, changing their account types to "limited" will prevent them from making system changes by default)
Note, the default groups who can change the time are Administrators and Power Users. If the person who should not be changing the time has an Administrator logon then you'll have to remove the Administrator group and add the user who is allowed to change time.
NOTE - DON'T MAKE THESE CHANGES WITHOUT FIRST CONSULTING WITH MANAGEMENT!