Again- no power in HP can stop magic before a spell is cast, that does not mean that no power can do this to HP magic. HP magic is limited by casting time to a higher degree than a lot of magic systems are, there's no reason that a faster system could not, in theory, block the HP wizard before the spell is cast.

I'm not saying that Sauron can do that, merely that your argument is logically flawed. I'm a math major, we tend to be somewhat anal about this sort of thing. So in set theory: {HP magic} is a subset of {fictional magic}, there does not exist any spell S or ability I that is a member of {HP magic} that can block a HP wizard (who's a member of {HP wizard}, which is a subset of {literary wizards}) from casting spells. But this does not imply that there exists no spell S in {fictional magic} that can stop a HP wizard's spell before it is cast.
*sorry about that, I can't help myself*

Again, I don't think that Sauron can do this, what he can do is reflect or re-route any spell, then shatter Voldemort's wand. Battle over. Just for the heck of it, I'm going to go into some real hard-core nerd stuff (not that this thread has had any of that or anything...)

Here's the breakdown of basic attributes of both sides:
Speed:
Voldemort can move quickly through magic, but other than that, he's probably not very spry, due to the lack of physical activity, being seventy+ years old and not exactly having ever been the athletic sort. In terms of ability to bring the pain in a hurry, V-boy's looking at a hefty 2+ second casting time for every spell.

Sauron is fast- he's one of the greatest melee fighters of all time, and suprise suprise, melee fighters tend to be fast. Check out the Iliad, the fight between Achilles and Hector, two of the greatest warriors of all time is over in what, two spear throws and one spear thrust? That's the kind of fast that Sauron's going to be. He can probably cover the ground on foot at a pretty clip, and at melee range is going to be, well, godly. This also means that he's going to very, very, very good at dodging things.

Toughness:
Voldemort is a seventy year old man with that pesky lack of physical activity etc. He'll fold up in one hit like an accordian under a cement mixer.

Sauron is destroyed by the demolition of a major land mass that reshaped the entire planet and in a solo melee with two three of the greatest fighters of all time. He's tough.

Magical Ability (general):
Voldemort is the greatest dark wizard in a hundred years. He's got the standard array of HP magical nasties at his disposal, plus probably some sort of extra uber-evil wizard stuff. Nevertheless he is defeated repeatedly by a child lacking full training. On the plus he has pushed the limits of dark magic to the limit (apparently because dark wizards are morons and nobody ever thought of multiple horcruxes before.)

Sauron is a spirit with, oh the entire life of the world of experience. He's created new forms of life, built fortresses that even gods take a long time to destroy, created an entire branch of magical lore, and forged the fourth most powerful artifact of all time. He can control weather, detect magic over very long distances and build indestructable buildings.

Magical Ability: Offensive:
Voldemort has AK, presumably Fiendfyre, torture, mind control, the usual bevy of school-type jinxes, some summoned weapons and presumably some other stuff, such as those silvery bolts Dumbledore throws around- although those may be unique to Dumbledore since he's the only one to use them.

Sauron's magical offense is technically unknown beyond his abilty to absolutely destroy people's minds. To keep things somewhat interesting, let's assume that Voldemort is not a complete moron and doesn't try to enter into mental conflict with Sauron, and, for lack of detail, let's not give Sauron any offensive magic at all. The only exception to this is the ability to shatter stuff, since its a known magical power.

Magical Ability: Defensive:
Voldemort has protego (I'm guessing, he never uses it), and probably some advanced dark arts stuff, but its never used, so I'm going to ignore it for now. Mostly he gets by because he's barely ever attacked. Voldemort never shows any ability to protect anything besides himself however.

Sauron can shut down pretty much any magic that's been thrown at him. However, even the greatest of beings make mistakes, so let's assume that Sauron can block 9/10 of incoming spells.

Territory: Let's assume a neutral ground, with some cover and obsticles, and full availability of power to both combatants. Assume that there's plenty of room to maneuver as well.

Likely combat range: From my previous analysis, Voldemort will have to be inside of a hundred feet of Sauron to be able to be at all effective. Sauron needs to be somewhat closer, say inside of ten feet, but if he manages it, its an automatic win for him.

Course of battle:
Voldemort apperates a hundred feet away from Sauron and casts some spell or other. Sauron dodges, moves towards Voldemort. Its fair to say that he can cover half of the distance between them in the time that Voldemort can cast a spell. Now they are 50 feet apart. Voldemort casts again, this time he's within range to keep Sauron from dodging so easily. Sauron counters, closes remaining distance. At the last moment Voldemort manages to disapparate 9f he's lucky, otherwise he's a grease stain. Sauron hears the loud "pop" and turns to face V again, moving let's say 20 feet towards him. Voldemort now has a max of 80 feet. He casts a spell, Sauron dodges, and shatters Voldemort's wand, then charges. Two seconds later, Voldemort is chased down and killed.
The end.