As an administrator, go into the control panel, administrative tools, Local security policy (or whatever you use for your domain security...I don't have any experience with that). Click local policies (or domain or somthing, again, I don't have any experience with that), User Rights assignment.
There should be an option for "Change the System Time" If you double click on that, you can just remove all of the groups except administrator and then all of the regular users will be unable to change the system time.