best response confirmed by foamyirishman (New Contributor)
Solution

Hi @foamyirishman,

 

I believe you're talking about Other Suggestions List. According to the article linked below, "You can't remove the entries in the Other Suggestions list or disable this list".

https://support.office.com/en-us/article/manage-suggested-recipients-in-the-to-cc-and-bcc-boxes-with...

 

However, I've tested it myself and a simple workaround seems to do the trick:

When an unwanted entry is listed under Other Suggestions, keep typing more letters in the TO 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 be removed from Other Suggestions as well.