To end a project so that no more work can be targeted to it, you must add an end date to the project.
Go to Worktime > Projects > Project administration.
Choose the right project from the Projects menu.
Add the ending date to Project ends -field.
Save changes by clicking "save project" at the bottom of the page.
NOTE! If a project has been set as a person's default project, they can still record hours to it even after the project has ended.
If start and/or end times have been set to a parent project, they will be copied to all sub-projects. If these dates are removed from the parent project, they will not be removed from sub-projects automatically but they need to be manually removed from each project.