Personally, I would probably try this:
1. Shutdown Bitcoin Core
2. add walletbroadcast=0 to the bitcoin.conf file to stop your wallet rebroadcasting these transactions
3. Start Bitcoin Core
4. Right click on the transactions and try: "Abandon Transaction"
If that works, it should "give" you the BTC back and you can then attempt to create a new transaction with a "proper" fee

NOTE: You'll probably need to shut down Bitcoin Core again, remove the walletbroadcast line from your bitcoin.conf file and then restart Bitcoin Core again to be able to send your transaction out
