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

1. Go to Projects and click the Actions menu to the right of the project you’d like to edit.
2. Select Edit to get to the Project Edit form.
3. Under Tasks, click the X to the left of each task to remove it (if there is no time tracked to it) or archive it (if there is time tracked to it).
4. Save.

Note: Once you archive a task from 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.