Before proceeding with the deletion of a server, it must be stopped. To do this, click on the stop icon (the black circle with the white square in it) that is on the line of the server that you want to stop.
Then, when stopped, you will be able to click on the X icon in order to delete the server. You may then confirm your request.
During the deletion of your server, the system disk that is attached to the server will be deleted by default. If you want to keep it, please check the box that says “Check this box if you want to keep the system disk”. The data disks are automatically removed, and will still be available to you from the “Disks” tabs. You can then delete them individually later.