Polls can be closed and re-opened now (link on the "[ Fill out Poll ]..." line of links on the poll page).
Ending Votes: LiveJournal polls cannot be closed.
-- Do Not Delete Polls: If you delete an entry that contains a poll, the poll will become inaccessible. You cannot create a new entry to restore an inaccessible poll; you must create a new poll.
-- Change Entry Security to "Private": This will prevent further votes by preventing everyone else from seeing the entry.
-- Edit your entry to indicate that voting has ended.
I think we could strike the last one definitely and the second one most likely - hiding the whole thing isn't most people's idea of closing anyway. Do Not Delete needs to stay whether in "Ending Votes" or somewhere else in the FAQ.