Use the drawing tools to put a vertical line in the plan and in the line
properties lock it to the date. A better way than a vertical line would be
to insure you have a final "Project Complete" milestone and all dependency
link paths terminate on it - while not every task in a project will have a
predecessor necessarily, every one will have a successor - if nothing else,
the Project Finished milestone. Then set a deadline (Task Information,
Advanced tab) on the finish milestone. Now MSP will "red flag" if you're
going past the deadline. You could carry it farther and put a deadline on
all the tasks in the plan if you like but I don't think that would be
particularly valuable.