You can delete projects and tasks, but only if they aren’t in use.
In most cases, we recommend archiving or closing projects and tasks instead of deleting them so a record of their data is retained.
To delete a project,
A dialog displays.
If the dialog says data has been logged against the project, click Ok to archive the project. If you don't want it marked as archived, click Cancel.
Refer to Adding tasks to your project for details on deleting tasks.
The only way to delete a project is to delete all data that's been entered against it from user timesheets and expense sheets.