If you have your seed... then yes, it would be VERY easy to "stop" this transaction from being broadcast.
Simply find your current wallet file (
http://docs.electrum.org/en/latest/faq.html#where-is-my-wallet-file-located), and rename it to "OLD_WALLET_DO_NOT_USE" or something equally obvious
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
Or better yet, rename it and move it OUT of the Electrum wallet directory.
Then install the latest version of Electrum, and regenerate your wallet using your old seed (File -> New\Restore -> Standard Wallet -> I already have a seed). As the transaction was never broadcast, it can't exist in your new wallet
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)