I think it is great that some stand up for the potential handicapped person that may need to use that parking spot that is reserved for just such a case. However, to bring it to ones attention that is not entitled to park there even for a moment, you may get a better response by doing it a little more tactfully, for instance try informing them nicely that they may not know it but they are in a handicapped spot that is reserved for only handicapped and as long as you are parked in it, it leaves it unavailable in the chance one may need it.
I have witnessed a-holes park in front of ramps up to the sidewalks at intersections and a guy in a wheelchair trying fruitlessly to get up on the sidewalk
and not be able to because of some ignorant a-hole that was blocking it with his/her parked car. It IS all of our business and that crap needs to stop.
So try to educate the uneducated as best you can and try not to get in an arguement over it.
can't we all get along