Sometimes user accounts and profiles that have been removed and deleted from AD still appears in the People Picker dialog box. You would expect SharePoint to remove these when they are excluded on the profile sync but unfortunately it’s not always the case.
The quickest way I’ve found to remove the account is as follows, however it is a very manual process and you might need to do this for every site collection\site (depending on how your permissions are set up).
Connect to your SharePoint SQL Server instance
Run “Select * FROM [UserInfo] where tp_login = ‘DOMAIN\USER'”
Take note of the tp_ID
Then browse to http://YOURSITE/_layouts/userdisp.aspx?ID=tp_ID
Remove\Delete the user