I feel your pain. I don't know how effective it is, though. The wise need no instruction and the ignorant can't be made to learn. More than likely the oblivious person doesn't even know you slapped the car, but just thinks "What is that noise?"
I have slapped cars myself when they've done something wildly stupid. You may get a lot of "you shouldn't do that" replies here, but I don't see a huge problem with letting somebody know you're there. I'm not sure it is a highly effective action, but in situations such as the one you describe there isn't really a very effective option available. Assuming they are able to connect the dots and know where the noise came from, it does at least let somebody know they came very close to you.