You'd have to have some crazy powerful magnet to do something like that. Think about sub-woofers in a single chamber box. Those magnets are plenty strong and not shielded in any way. They don't interfere with each other no matter how close they are put together. I've seen plenty of subs face away from each other with their magnets directly behind each other. Reverse mount bandpass boxes for example. I just don't see a ordinary thief carrying around a electric super magnet to defeat sirens. But, I don't really know... that's a pretty crazy idea.
