The best thing we have is the date of creation of the ofended website. Since the offending website is always created later, google and other search engines know wich one is the offending one and will penalise only them and not you.
The legal question is much more difficult. There are people around the world on the internet and from some countries that would not be much concerned with a letter threatening with legal prosecutions. In this kind of situation it may be better to speak with affiliate managers. They are in the best positions to take action and suspend accounts.