Deleting a project in HumongouS.io is a safe operation with regard to your actual database. When you delete a project, we only remove the metadata (connection information, collection configurations...) we've saved on our end about your database.
To delete a project, click on the ellipsis icon at the right of the project name and then click on Update project.
A new window will pop up. If you are the owner or have the permission to delete the project, you should see the Remove database button at the bottom left corner.
Click on it to remove your database.