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.