If you decide to no longer continue a project series (e.g. you're discontinuing work for a client), stopping future entries is simple.
While in the Projects page, open the project.
In the top right corner of the screen, click the View Series link in the Associated Series box.
Click the Edit Series button.
Under the frequency drop down menu, choose Custom.
Uncheck the Indefinitely box and choose an end date.
Click Save.
Click the blue Save Changes button.
You should now see the series ends on the last entry before the new end date!