I am still trying to figure out what type of message this is that would come from your Optus account and land on your desktop of your pc. Perhaps you could explain further as the only thing that comes to mind is an email message. Deleting emails without opening them is simple enough however I would need to know what pc and mail program you are using if it is in fact an email
I understand but need to know the email program and computer operating system. If it's a Windows 10 computer and its using Outlook email for example?
Typically deleting an email without opening it on a Windows machine can be done by right mouse clicking the email to bring up the additional menu then left mouse clicking delete. It varies with the email program and the operating system as Apple Mac don't have the right mouse function.
You also should be able to highlight the email in question with one mouse click then go to your top menu and find delete. If you have the preview window on this will unfortunately show the email. You may need to turn preview off as well.