1. Check to make sure OWA is not disabled (Exchange Admin, recipient, inbox and make sure OWA is enabled)
2. Go to outlook.office.com, login with email address and regular password
3. Click on calendar, grear icon, and click on the link that say view more settings
4. Click on event from email, and from the list of events, you can select the one that you do not want to show in calendar (Do not add to my calendar)