How to remove Gravatar from Username column

How to remove the Gravatar image from Username column in the All User admin page?

Since there is no special avatar column to unset (the avatars are inside the username column), you could try instead to hide the avatars via css:

function hide_avatars_wpse_94126() {
    if(!current_user_can('manage_options')){
        // hide only for non-admins
        echo "<style>.users td.username img.avatar{display:none !important;}</style>";
    }
}
add_action('admin_head-users.php','hide_avatars_wpse_94126');

where they are hidden for non-admins.

The result will be like this: