I run into this scenario every day. Someone piddling along in the passing lane and no amount of people passing on the right seems to affect them at all. I have tail gated them to get their attention, most of the time with out any success. I have honked the horn at them and gestured for them to move back to the right lane. Most just stare at you in their mirror as though you are crazy, some have even slowed down. Thank God I eventually get around them, they usually just continue driving in the passing lane until they are out of sight.