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 3 dots menu at the top of the screen and click them to open up the menu. Once the menu is open, select end series.
Pick the end date for the series and confirm
The series now has a scheduled end date. The series page will update with a message at the top of the screen indicating the series has ended. The associated projects section will also be updated and future instances past the scheduled end date will no longer show up.
NOTE: You cannot edit the series once it has a scheduled end date. You can edit an individual instance in the ended series though by clicking into the specific due date and making edits as needed.