Delete Entries in the Other Suggestions List in Outlook
Introduction
This article has its origin in several question posted in TechNet and Tech Community forums (like this thread: https://techcommunity.microsoft.com/t5/office-365/outlook-365-suggested-email-addresses/m-p/959420) and will provide you with a workaround.
Question/Problem
It's quite easy to remove one or more entries in the Recent People list in Outlook. After beginning to type an email address or name in the To, Cc, or Bcc field, Auto-Complete suggests the matching entries. Hovering the mouse over the entry and clicking the "X" (Delete) icon to the right of it will simply remove that person/address from suggestions. However, the same "X" icon doesn't appear when it comes to Other Suggestions section and it doesn't seem to be feasible to remove entries in there.
https://support.content.office.net/en-us/media/532812e5-d776-46d4-905c-4e2082f5b9d8.png
Solution/Workaround
When an unwanted entry is listed under Other Suggestions, keep typing more letters in the same To, Cc, or Bcc field. After adding a few more matching letters, the entry under the "Other Suggestions" list will be moved up to the "Recent People" list, and you'll be able to press the “X” at the right of that entry. By that, it will also be removed from Other Suggestions.
Conclusion
Even though it's stated that "You can't remove the entries in the Other Suggestions list or disable this list." in the article linked in the "See Also" section of this wiki, this workaround will work just well.