When I join a duty and the dps is beyond terrible (based on time to kill mobs / threat meter built into game), I just leave and take the 30 minute penalty and go do something on my alt or Q as a full group instead. Let the bad DPS sit in Q for another Hour.

No reason to try to fix people when they are bad, it takes little effort to be decent. They just aren't trying. Ignore and move on, any time spent otherwise is time wasted.