Step 1: Login into portal.office365.com
Sign in with your Login ID and Password.
Step 2: When entering account click on Admin as shown into the picture.
Step 3: In the left navigation pane of the Microsoft Office 365 admin center, click Exchange.
Step 4: After click on the Exchange a new window will open, in the new window click on the Recipients that is left side of the page then double click on the User.
Step 5: Now click on the Hide from the Address List checkbox and click on the Save button.