Especially in the past using "Sie" was a sign of respect, which is the reason why teachers are still adressed with "Sie" by the kids but the teachers uses "Du" to address a child. In particularly lopsided social relationships (like factory owner to worker) it wasn't uncommon for the superior to use "Du" and the inferior to have to use "Sie".
The point here, Gandalf using a "Sie" here would have been him being polite and respectful to the Balrog.
EDIT: also, yes "Du kannst nicht vorbei." is a direct quote from the Carroux translation. But Krege uses "Du" for adressing the Balrog also. I am not completely from the sure which intentions you care about here: original translator or movie translation? But in either case it would be extremely weird for Gandalf to use a polite form here and "Du kannst nicht vorbei" is a fairly direct translation of "You cannot pass." So it's unlikely that the original translator would go into any details on her intentions here.