Since this has been discussed in the q&a board, I decided to post some info on this issue here, so it would be easy to locate.
We will use a board named "The Forum" for this example.
If you have a whole bunch of posts on "The Forum" and you want to turn these posts into an archive go into the administration area and choose "Edit a Board". You can then select "The Forum" and then rename it to "The Forum Archive" also set it to Read Only. Next create a new forum called "The Forum". Now you have a brand new board called "The Forum" and a read only archive called "The Forum Archive". You can do this as much as needed.
In some cases this may never need to be done, either because you expire all threads after a certain date, or you only keep important threads and expire all others. In either of these cases you may be fine in keeping everything on one forum.
In a future update you will be able to set access levels to users and boards, so if you do decide to go the archive route and you have a ton of boards, you will be able to use the access levels to make only certain boards show up for certain users.
'If it's worth doing, It's worth doing loudly'