Again, using WPA2 with a good passphrase will do just that. No need for useless things like hiding the ssid and mac-address filtering. Those things are called security through obscurity.
About 99% of those scanners will pick up ssid's no matter if they are hidden so that would make hiding the ssid completely useless. Only the unsecured networks are interesting because you only need to connect to get on it.
WPA != WPA2. WPA2 is a lot harder to crack but then again both WPA and WPA2 are easily crackable when you're using a very short and simple passphrase. The better the passphrase the longer this will take. Most people (like the one you're talking about at the start of your reply) will not even bother connecting to a secured network since they don't know the passphrase. If it's the hackers you worry about then you need other measures (IDS and things like that for starters).
Which is why just using WPA2 with a proper passphrase is sufficient.
In your point of view that sign would be bad since the thief would know what alarm installation you have and therefore would be able to compromise it. The fact that thieves would smash the window and nick the radio in a couple of seconds goes unnoticed. Thieves absolutely don't care about signs, they care about getting that radio out of your car as quickly as they can.
What you're advocating is called security through obscurity. What everybody else here seems to be advocating is quite easy: use your brain; think about what certain things do and if they really are useful. In this case using WPA2 alone is for the random kids AND more interested kids. Hiding the ssid will not secure you from the random kids nor from the more interested kids as, like I said many times before, nearly all scanners will show the ssid no matter what. That makes it as useful as a car without an engine.
