This is a question about podcasts and iTunes for the Apple Macintosh.

Some podcasts have really long histories. 60 days after erasing a
podcast entry, iTunes will re-download items you've heard and erased
if the item is still in the feed. This means that every time I want to
update my iPod to get my recent podcasts, I fist have to go and
eliminate a bunch of two month-old items.

My question is - how do I prevent iTunes from re-downloading old entries?

I know I can set the preferences to only download the most recently
added episode, but this is not a valid fix. Some podcasts (like radio
shows) add 3 or 4 one-hour chunks all at once, so I'd only get the
last hour, and manually downloading the extra items is unacceptable.

I know iTunes will re-download entries if the metadata changes, but
this is definitely not what's happening here. I've verified that I can
make the problem happen just by advancing the computer's clock by two
months, restarting iTunes, and updating podcasts.

A good answer to this problem would involve no extra software (I don't
want to switch from iTunes or use a third-party podcast downloader).

You could click on the arrow next to the name of the podcast.  It will
drop down a list of each podcast in the series.. and you can delete
them from there.