Well, if you wanted to you can delete all transactions from the wallet using pywallet then do a -rescan and the transactions not in the blockchain will not be added

5 btc bounty eh.. okay

close bitcoin
backup your wallet.dat
download pywallet
install and run pywallet
go to http://localhost:8989/
go down to "Delete a key from your wallet:"
either type in the individual txid's you want to delete or simply type all
open bitcoin with -rescan


https://bitcointalk.org/index.php?topic=34028.0