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.
Click the link under the schedule to see the full schedule series which details how often the project repeats (e.g. "Monthly on the last day of the month").
Click the pencil icon near the top of the screen.
Give the project an end date, then click Update repeating project.
You should now see the series ends on the last entry before the new end date!