You can’t mark a task as complete, but you can remove or archive a task from a project. To archive a task on a project:

1. Go to the main Projects page.
2. Click Actions > Edit to the right of the project you’d like to edit.
3. Under Tasks, click the X to the left of a task to remove it (if there is no time tracked to it) or archive it (if there is time tracked to it).
4. Update project.

Once you archive a task on a project, you won’t be able to track time to that task (on that project) or to edit the time tracked to that task/project combination. The hours you’ve logged to the task are unaffected though; you can still report on and invoice for time tracked to archived tasks.