There is only one way to stop the metal thieves and that’s to target the dealers that purchase scrap metal! It’s not rocket science, it’s common sense.
If the thieves have no-one to sell it to it would be useless to steal it and if all scrap merchants who are aware that some metals that’s brought to them for them to purchase are, for instance, railway cable or roof lead and copper then they know it’s unlawful for them to purchase it.
They should call the police if they feel something’s wrong and be instantly shut down and prosecuted if they take these items knowing them to be illegally obtained and scrap collectors need to be licensed.
A regular visit to all scrap merchants for a surprise search will be the only way to stop this deplorable theft.