Hi Chrisst,
I did a test by deleting the Suggested Contacts from OWA and restart the MAC. This Address list disappear from both of them. You can log in to OWA through http://mail.office365.com/ with your account. Click People and expend Other Contacts. Right click on Suggest Contacts and click Delete.
Please let me know whether this works for you or not.
Thanks,
Yang Yu