Comment: B. Carfree, in the case you cite, there is not much preventive that can be done unless you have the resources with which to electronically scan your environment, or, at the lease, suspect photographic monitoring is happening and do some snooping of your own. Otherwise, preventive steps can be taken as I've described in my letter.
There may be instances wherein the subject has agreed to nude and sexually explicit photography so as to have a record for posterity. In that case and in the instances included in my letter, the subject may need to be apprised of the risks inherent in such endeavors. Then he or she can make an informed decision as to whether or not a photographic record of nudity and sexual activity should be created.