New to MYOB - I would like to set up MYOB with the ability to use "cut off dates" example: I am closing June, and no longer want anything to be entered to prior month, how do I do this ? Is there a way to tell the computer, no matter what do not allow prior dates to be used ?

Yes. Go to Setup: Preferences and then the Security tab.
"Lock Period: Disallow Entries Prior To: xxxx"