Mace would be an acceptable, NON LETHAL way of deterring a dog if they're that scared. In most cases though, seems as though a dog barking alerting his owners to strangers = an aggressive dog that wants to maul you nowadays.

Here's another example. Notice the footage of the bulldog happily mingling with the children of various ages in the backround.
Idiot.
http://www.youtube.com/watch?v=CgH0Oy3z1vo