Projects that are no longer needed to be kept in your workspace can be moved to trash. You find this option in the context menu (three vertical dots) next to the project title.

To see which projects are in your trash, select "Trash" from the sidebar.
As workspace member you can restore projects from trash and delete them forever.
Once deleted, the creative within the project no longer counts against your storage quota.

Keep in mind, if you "delete" the project from trash, there is no option to restore the project.