If you want to prosecute people for trespass based on DNA alone, consider the fact that is a regular supply of DNA-keyed evidence available for free in the ashtrays of many bars, convenience-store trashcans, and so on. Indeed, if you get into a dispute with a vindictive neighbor it's probably not hard for them to get your DNA from your trash and use it to cause you a serious legal headache while you establish your alibi.