In the German translation of Tolkien's "The Lord of the Rings" one does nowhere find  the modern polite form *Sie*, but the old-fashioned *Ihr*. Why is that? I think the use of *Ihr* wants to indicate that all this happened a very long time ago.

The phrase *Du kannst nicht vorbei!* is taken from Margaret Carroux's German translation of Tolkien's books. See Band 1 (Die Gefährten), Buch 2, Kapitel 5 (Die Brücke von Khazad-dûm). In the Klett-Cotta Edition from 1987 you can find it on p. 399. 

In my opinion it would have been better to say *Du kommst nicht vorbei* instead of completely omitting the verb *kommen*. Anyway, your main question is why the Balrog is *geduzt* by Gandalf. The books definitely uses both *Du* and *Ihr* to adress persons (i.e. human beings, hobbits, elves, dwarfs) and so do the films. See [here](http://www.filmzitate.info/index-link1.php?link=http://www.filmzitate.info/suche/film-zitate.php?film_id=567), for example "Ihr seht viel, Eómer, Eómunds Sohn, zu viel". Even Samweis uses "Herr Frodo" and "Ihr".

I suspect that other creatures are never adressed by the polite "Ihr", especially if they belong to the "evil empire": They do not deserve respect or reverence.

**Update:**

Here is the [English original text](https://www.goodreads.com/quotes/546473-the-balrog-reached-the-bridge-gandalf-stood-in-the-middle):

> There was a ringing clash and a stab of white fire. The Balrog fell back and its sword flew up in molten fragments. The wizard swayed on the bridge, stepped back a pace, and then again stood still.  
**'You cannot pass!'** he said.  
With a bound the Balrog leaped full upon the bridge. Its whip whirled and hissed.

This explains the translation *Du kannst nicht vorbei* .