IMHO, if a thread is answered, there is no need to close it because people tend to just express gratitude and walk away anyway. Occasionally there are some necros, but a.) overall it is not a huge problem and b.) sometimes it helps to identify the spammers early on, which is a plus?
And if a thread is not answered, there is no point to close it, too, because someone might walk-by and provide a relevant answer after an indefinite frame of time, perhaps after he himself encounters a similar situation or solution. Never know with these things.
Just foster an environment friendly to discussion and let things take their natural course. This site already has good SEO
. Most users are generating good content now that the spammers are pushed off a dangerous cliff with sharks waiting down below. And content is king! We don't and shouldn't have to make changes for the sake of SEO