What happens if while off-line, you open MIST switch it to the test network, close it, go online, open MIST and fire the transaction?
I have a similar problem with an account I created that is stuck doing nothing, I delved into all the files via deep text search looking to delete that specific one. I have not followed through though, when I do and if it works, I will report back.
While I wait for my MIST to sync:
remove and save somewhere, the corresponding keystore file for that wallet. It will UTC-- the address of the wallet. This should remove that from MIST when you open it up. Question is will it flush out any pending transactions that are written somewhere else? No wallet nothing to send. Close MIST put back the keystore file in the folder, the account will be there again, hopefully minus any pending transactions. If not
It is or has been written elsewhere by GET? If so you must be able to delete it direct from GET? I don't know the commands but surely someone can input if this is the case, how to remove a pending transaction from GET, if indeed it has written and saved that.