Try ctrl clicking on the problem podcast show title and then select "Delete from Library"
One of two things should then happen, you will have no podcasts under that show - so just go to Podcast Settings and change the subscription because hopefully it shouldn't be locked anymore. Or if your show disappears go back to the store, search and add it back by subscribing, again once its back go to Podcast Settings to confirm it's no longer showing the lock.
Basically I've found to clear the "lock" you have to find some way of applying "Delete from Library" to the problem podcast show (ie show title - in my case "AppleInsider Podcast).
Not sure why this is happening because I've tried unsubscribing on my iDevices and the Mac as well as deleting shows but when added back I always got a couple with this issue.
One thing I did discover is that while iTunes showed the lock in one view it wasn't in the other (ie not locked in Classic List), also looking on both my iPad and iPhone did not show the problem - so it might be something to do with iTunes List view.
I've had other issues related to syncing which I won't go into now, but to try and resolve them I decided to delete the Podcasts app from both iDevices then add it back - read somewhere that removing clears out data that could conflict with what iTune has stored, early days yet though.