Scheduling and managing events using Outlook.com calendar
I have the same problem. The Microsoft Suggestion spam calendar item is only in my iPhone Hotmail account calendar. It does not appear in the iPhone Outlook app Calendar. I think iPhone/Siri is seeing these spam calendar items when it syncs junk or trash and adds it to the email account Calendar (hotmail in my case). Some screen shots of the spam email from the IPhone and Outlook apps. Permanently deleting the email item does not remove the iPhone calendar Microsoft Suggestion spam appointment.