Perhaps you could just close the forum and leave it as a searchable archive. I'm sure everything has been discussed at some point, but the point of a forum is to discuss things. Ideas and products change as does consensus of opinion, new members bring fresh ideas, so although a search will often bring up information it is not an exhaustive source of information, neither is it a conclusive consensus of opinion, and often you have to trawl through pages of off topic banter to find the few good posts. So very often new threads generate new information and ideas. That is the purpose of a forum and without new threads it simply dies.
Rant over