They could use microdots. These are impossible to entirely remove and ensure that thieves or fences can be caught, even if they chip the copper. Though once the avenue for turning scrap into cash is curtailed, the problem will move elewhere, unfortunately.
http://www.hpicheck.com/theftguard.html

Last edited by Alan Belson; 06/14/12 09:59 AM.
Wood work but can't!