Everytime I see one of those dogs I look for the nearest brick or club to beat them with if they attack me.
I saw an ad for a whistle that is alleged to drive all dogs away. However, I doubt it works.
A couple of years ago, some idiot of a council person in the city of SA proposed a "dangerous dog" ban for certain breeds within the city limits which fortunately did not get passed. The list not only included the usual suspects, but also Malamutes, Huskies and Great Danes. My Siberian Husky is an excellent watchdog, but she is an exception to the rule for that breed-Huskies get stolen all the time out here because they will go to any human that comes to their fence-we keep ours indoors when we are not home for that reason. I've never met a vicious Malamute or Great Dane, either, but I've met some that will bark and snarl if you go on their property uninvited, like my Husky does.
We had a Chow that everyone who didn't know him was terrified of, because he looked and sounded totally vicious if his home or humans were approached uninvited. We did not let him interact with other people's kids for safety's sake, but he was always gentle with ours and protected them as well as our German Shepherd had in their younger years. That "vicious" Chow died of old age at 17, and was one of the most wonderful dogs I've ever known.