Someone who gets it!
It's not so much agendas as failing to see them combined with the "We've got the facts on our side, how could we possibly lose?!" mentality. Switching back to creationism for a moment, good old Dawkins supplies us with a salient example: "If they’ve [the creationists] been told that there’s an incompatibility between religion and evolution, well, let’s convince them of evolution, and we’re there! Because after all, we’ve got the evidence." That'll make any cognitive scientist, historian, anthropologist, etc. bust a gut and die of laughter.
There are some tools to help with this, of course. I know I pulled that one with a discussion on slavery in the Bible, and the guy claiming "it doesn't really endorse slavery because it's a different kind of slavery" shut the fuck up pretty sharpish after that.
But speaking of the hard green and politics stuff, there is the episode of Penn & Teller's Bullshit! on Greenpeace (IIRC, of course) where someone actually asked a Greenpeac protester about GM foods. Even if you haven't seen it, you'll be unsurprised to hear it was derailed to "capitalist corporations are evil" in about 3 seconds flat.