is there some simple way to archive one's journal?

Yep, just click on the "report post" button () and ask the mods to remove your journal. Just remember that our policy is one journal per member, so if at any time you wish to recommence journalling, you just need to contact us, and we will return your journal to the Journals forum.

I actually wanted to save the postings offline, not make it unavailable.

The best way to save your posts offline would be to click on "Show printable version" (under Thread Tools) then copy and paste onto a Word document or similar, or just print out the page.

Or do the printable version then do "Save As" from your browser.