It's an interesting debate as to when things should come off the table. My usual metric is that respected people start to refuse to talk about it - at some point topics just stop being interesting to them and that's the point that it generally means that it's over. If for no other reason than because you know no one is going to be generating new evidence as they sure ain't going to be getting any grants!
Crank science is different. I guess that The Media and capitalism are the problem. Homeopaths are probably never going to go away because there is money to be made, and The Media are probably going to keep reporting on it because it sells.
Perhaps there will come a point (like with UFO's) where interest will simply dissipate and it'll stop being a story.
Until then we have a problem - the process is the story, just like for AGW. Why aren't there grants for this? Why do wise professors simply dismiss it? What on earth are we doing supporting these massive drug companies with their expensive and somewhat poor therapies? We need to be aware that the processes of science (everyone I know thinks this is just uninteresting, therefore there is every reason to believe that anyone working on it is wasting there time - unless I have reason to think otherwise - so mehh) doesn't have the effect that we anticipate it to; so lots of careful, calm, open minded rebuttal based on cast iron facts and offers of changes of opinion if things can be proved other wise would be good.