I know iTunes has the ability to auto-delete podcasts after they've been listened to, but that function has never worked for me.  When I click "Settings..." in the Podcasts tab, I have "Episodes to keep" set to "All unplayed episodes", which to me means all played episodes will not be kept.  However, in real life, this doesn't happen.  Thankfully there's an easy way to fix this:  Right-click a podcast and click "Allow Auto Delete".  Or you can press Ctrl-A to select all, right-click, and select "Allow Auto Delete".  The one caveat is that podcasts are only auto-deleted after a refresh.