Deleting a project is permanent, and only the owner can do so. If you’re seeing “Leave project” option then you're not the owner of that project.
For the team projects, only owner of the team and users with admin rights can delete projects for that team.
Go to the Projects page.
On a project card click More Options icon > Delete.
On the popup checkmark all terms for deleting.
Click Yes, Delete Project.
Note: You can delete projects only from the web app. Deleting Projects will erase project version history, all users on the project will lose access, and all shared links will be inaccessible.