It seems that an Expired Deals forum would be beneficial. After a deal expires, it gets moved there, where discussion can still take place on it, but people who are looking for new deals won't mistake it for a recent post.

That would be a lot of work for the mods. These guys work for free, they don't get paid.

When a deal expires, it will eventually sink into obscurity. Before that, sometimes there is a lot of discussion about a deal, and therefore stays afloat. The OP or a MOD can stick "expired" in the subject, and that should/does suffice.