I recommend not being a jerk about it. Endangering their registrations is not going to put you in a good bargaining position. The registrar is happily being paid for these domains, and just because an address bounces for you doesn't mean it bounces for everyone (unless it's an unknown user). In any case, these issues are easily resolved in the registrant's favor, and they're not going to be in a good mood if you spoil their day. If you can't get a reply, pick another name and move on.
Also keep in mind that not all domains are used for the World Wide Web. I regularly get inquiries about domains I own because "they aren't being used" when, actually, they are critical to my infrastructure and just don't have any web sites associated with them.