In the iTunes 'Downloads' section, select and delete any downloads. Quit iTunes.
Look in your iTunes folder, where the audio files get stored. For example ~/Music/iTunes/iTunes\ Media/. Here, you should find a 'Downloads' folder. Inside it, 'Podcasts' (assuming that the file you're downloading has the media type of podcast).
Inside here should be a folder with the name of the podcast subscription with '.tmp' appended. Inside here should be the partial file that you've downloaded. Trash the folder and its contents.
Restart iTunes. Note that iTunes can be a bit sticky when it comes to automatic downloads (e.g. I am asked to log in twice when launching iTunes). The actions above should result in there not being a 'Downloads' on the sidebar. If for some reason, there still is, select and delete the items there again.