You can either delete or archive projects. Archived projects maintain all of your history and can be unarchived at any time, but don't count against your active project limit. There are two catches however, archived projects can't accept new issues or comments and archived projects still count against your storage.
In order to archive a project, go to the project you want to archive and click the project settings gear.
Next go to the "Archive Project" page in the sidebar.Lastly read through this page then if you decide to archive your project just click the "Archive Project" button at the bottom.