I would respect the sign. I once canvassed a house where the old guy dragged himself to the door to answer my call; he must've had severe arthritis; and when I asked if he wanted a window cleaner, he looked gutted. He didn't want one, nor did he look like he appreciated being dragged to the door.
Maybe some people in similar circumstances stick 'em up, maybe others do it because they're grumpy, maybe others because they're vulnerable people.
I'd give 'em a miss, personally, there's lots of doors without these stickers.