My point on the 'disputing' is that they should call the cops then that way the cops can all but say, "You're being a f*cktard, get a life and leave this guy alone."
If someone did somehow manage to get one when they shouldn't have, I'm sure with some proof there would be a way to have it taken away from them. But it's not my problem, so I'm not gonna waste my time on it. I see a decal (or plates) or if they have an obvious disability (even without the decal/plates) and to me, they qualify so I don't worry about it. If I see someone who looks as healthy as an Olympic athlete and doesn't have any decals/etc, then I'm gonna report it. If they shouldn't be parking there, maybe paying a $100+ fine will help them to remember the next time.