This is a handy little feature to manage your tasks.  Go to Manage > Tasks.  Click on any task – you’ll see a little trash icon next to the task name.

The trash icon does 2 things: if you have not tracked any time under this task, it’ll let you remove the task entirely.  If anyone in your account has tracked time towards the task, then it’ll offer you the option to archive the task.

What does it mean to archive a task?

First, you won’t be able to track time to that task anymore.  Second, you won’t be able to add that task to any projects.  If you change your mind – simply go to Manage Tasks, click on “Manage archived tasks”, and you’ll be able re-activate them.