Deleting a podcast off of an iPod Touch
Try to delete it directly on your iPod.
It may be in the Podcasts app (Downloads tab) if the podcast's download is still not finished, or in the Music app (Podcasts tab). To delete a podcast, you just have to swipe left or right on it.

PS: Unfortunately, I don't know what iOS version do you have installed and I cannot comment to specify this, so I have written these tips for iOS 5.