How do I keep Last.fm from scrobbling podcasts?
Open the Last.fm application.
Menu item "Last.fm -> Preferences"
In the Scrobbling page, edit the "Scrobble Directories" and make sure that your podcast directory is not selected. If you have your iTunes library in the standard location: Expand the directory tree to find /Users/you/Music/iTunes/iTunes Media and make sure that the directory Podcasts is not selected.
Review the other directories that are selected, for example you might want to make sure that the Audiobooks directory is unselected.
OK!