Old posts serve as great "food" for search engines and search engine exposure; if you rely on Search Engines for traffic this is something I would completely avoid.
As you mentioned, CloudFlare is an excellent option and will stop a significant amount of bogus/abusive/spam traffic when properly configured.
We don't have permissions for this however you could make an archive forum with the only view access being members (through permissions) and move old content into said archive forum.
