It is not possible to delete Projects and Tasks but you can easily archive them.
In order to archive a Task, you first need to mark it as Completed. Then a button 'Archive' will appear next to it.
To archive a Project, you need to mark all Tasks as Completed. Then a button 'Archive' will appear next to the 'Pen' icon on the top of the page.
You can easily access your archived Projects and Tasks by clicking on 'Show Archived', as well as unarchive them in the future.