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:
