Most people have at one time or another hitched their internet connection on to someone else's wi-fi signal, or have had others use theirs. Annoying? Yes. But it could also get you in trouble if the snooper is engaged in illegal activity, says The Working Guy's Christopher Null.
It's not just about slowing down your connection; while they're downloading Mad Men via bittorrent, you could be on the hook for their actions.

Wireless security and encryption systems are fraught with problems and insecurity, and other methods to restrict your signal to a small area are cumbersome at best.
This is particularly a problem if you live in an apartment building and there are a large number of wireless customers crammed within a small area.

Enter anti-wi-fi paint. The paint "contains an aluminium-iron oxide which resonates at the same frequency as wi-fi - or other radio waves - meaning the airborne data is absorbed and blocked. By coating an entire room, signals can't get in and, crucially, can't get out," says this BBC article.

There are many other applications, including medical and commercial privacy and security. I'm a big fan of the potential to block phones from ringing during a movie (you could block signals from outside so people wouldn't get calls).
Electronic jammers that actively block wireless signals are illegal, but passive materials that prevent wireless signals from getting through are not. Since the wireless-blocking paint can also block the lower-frequency signals that cell phones use, addled mobile junkies would have no outlet for reaching the outside world.
But is it really worth it? Sure, if you need a new paint job, or, as my IT husband points out, if you're a hospital, prison or secure government building where you can justify the additional cost of having to repaint exterior walls in the name of security or patient care.

But the "thought of having to redecorate a building in order to provide Wi-Fi security is more costly and complex than the security functionality available in even the cheapest of Wi-Fi access points," and you could always just disable wireless.