Sorry to hear that you think your cats are being stolen. Do you have suspicions that it is a particular person? or are they just disappearing?
I'm not sure where the law stands when it comes to cats but maybe contact your local CAB and ask them for advice.
Have you microchipped your cats? I would think that would prove the cats are yours but if they decide to live elsewhere I'm not sure if there is anything you can do sadly just hope that they come back to you.
If you get another cat or yours should return it may be worth while microchipping them.
Sorry to not be much help but if you explain your situation a little more then someone on here may be able to help you further.
I would agree with wingnut.......cats are notorious wanders,particularly if male. Unfortunately there are too many unscrupulous (sp?) people who would happily take a cat-especially if it is pedigree. I see it all the time in my local paper-Bengals,Birmanns,etc that just 'disappear'....it is sad. Good luck....they may still turn up.