Also, that option sounds scary, and like it does much more than drop unconfirmed send transactions.
I'm pretty sure it DOES do more than just drop unconfirmed send transactions:
-zapwallettxes= Delete all wallet transactions and only recover those parts of the blockchain through -rescan on startup (1 = keep tx meta data e.g. account owner and payment request information, 2 = drop tx meta data)
It literally deletes ALL the transactions stored in your wallet file... then rescans the blockchain looking for transactions relating to your wallet. This is why unconfirmed transactions "disappear" from your wallet when you use this command... as they're not in the blockchain.
However, this does not remove transactions from mempools... and it doesn't prevent anyone that already has the raw transaction from rebroadcasting it... All it does it remove it from YOUR wallet.dat