If I were a mod here I would do one of the following:
#1 if there's a question that's answered in a sticky, i'd lock the thread and link to the sticky. (as a mod, it would be my responsibility that the most frequent questions and answers where covered in a sticky anyway, regarding say dosage of creatine)
#2 if there's a question that's recently been discussed, meaning first or second page, i would lock the thread and link to that.
#3 if there's a question that's been up before, but not really recently, i would let the thread be since a board where people keep saying "dude, use the search!" creates a unfriendly atmopshere.