Might sound like a daft thing to try but i know it honestly works.....
Put up a poster like a wanted type thing saying these are of no real use as they dont work and if who ever took them wants to earn some cash to ring you and you will pay them say £200 and not mention who why what etc to the law, but soon as you find out who the person is just get them bashed and take them back anyways.
People are that thick they actually go for it for a cheap buck lets hope they dont use forums or have any idea about markers