As far I know of, there isn't a way to close it off. In my mind, the reason that hubpages does this is to encourage continued dialogue and conversation about a good question. Also so that people that may have a similar question in the future can access the information that your question brought out. Efficiency of questions i guess.
Any kind of good information is a good thing to have access to, especially if it's insightful and/or can help others. I hope this helps a little bit. Take care and have a wonderful day!