If you have deleted a user from Office365 and it's still showing up in O365 Backup, you can follow the steps below in order to remove that user from the list.

1. Firstly, this user must not have any backup data. If you wish to completely remove the user, firstly you need to delete all backup data for the User, as documented here.
   
   
2. Once Backup Data has been deleted - allow some time for data to be deleted completely. After that, you should now see the option under [Backups] > [Users].
   
   Once you have located the user, click the options menu next to the user and then click on [Remove the User from the list]