If a pistil gets pollinated it will oftentimes turn brown quicker than the others, but that's not always necessarily a sign that it has been pollinated... it might just mean it was an older pistil. I wouldn't chop the plant down, especially since you're so far into flowering, but I would take it aside and get a good look at the bud sites with a magnifying glass. If you see any tiny banana shaped pods hanging down from the buds, pinch them off as soon as you find them. That should curtail more pollination until it's time to chop it down.