In Harry Potter and the Deathly Hallows Part 2 movie, why do we see Harry and Voldemort punching, hitting and throwing each other? That is Muggle fighting, which I think was not acceptable in the Wizarding community!
The first example of such muggle fighting (by skilled wizards capable of doing entire fights with magic) we saw in the canon was Arthur Weasley vs. Lucious Malfoy in the beginning of the second book in front of crowd. This was also labeled as uncivilized act by many. Underbench fight of Ron during Quidditch match in the first book was also uncivilized, but they didn't have much choice as they were first year (I suppose you asked your question for adult skilled wizards).
Yes, punching, hitting, and throwing weren't respectable acts in Wizarding Community, but when you loose your temper, your wizard/muggle differences no longer matter. You get down to your basic nature and you act as a wild animal who has four limbs which can do lots of things.
Note: Don't discriminate Muggles. Punching, hitting, and throwing is not respectable even in muggle communities.
There are examples of Muggle duelling (McGonagall's words, not mine, from the book Order of the Phoenix) throughout the movie series.
- In Prisoner of Azkaban, Hermione punches Draco in the face when Draco makes fun of Buckbeak's impending execution.
- In Goblet of Fire the Weasley twins have a full-blown Muggle-style fight after they try and put their names in the Goblet of Fire by using an aging potion, and the potion backfires.
- In Order of the Phoenix, Grawp apparently continuously beats the heck out of Hagrid, although this is offscreen (feel free to disregard this example).
- In Half-Blood Prince, Draco stomps on Harry's face when Harry spies on Draco on the Hogwarts Express, breaking his nose.
There are a lot more examples of Muggle duelling in the books, but as you're asking about the last movie specifically, I kept my examples to the movies. The books suggest Muggle duelling is frowned upon in the wizarding community, but is not forbidden. As Muggle duelling appears in the movies throughout the series, we can conclude it's not forbidden per se, but it does seem a bit unusual for, say, the Weasley twins, who were raised in the wizarding community, to resort to Muggle duelling. It makes more sense to me to see Hermione, for example, engage in Muggle duelling because she was raised as a Muggle and is Muggle-born. Hermione does slap Draco in the books, so she's not above it.
Regarding Harry and Voldemort, I think sometimes people just have visceral reactions that cause them to act out of character, or to employ means of fighting that might not be expected.