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.
Go to the Start Date field. When you hover over to edit this field, you'll see the editing options for both the start date and the schedule appear.
Click into the schedule dropdown menu and select 'custom'
Make your updates to end the series by deselecting the 'indefinitely' box and entering the end date of the series.
Click Save to apply changes
You should now see the series ends on the last entry before the new end date!