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, 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:
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 .