rm -rfv ~/Library/Ethereum/testnet/geth/chaindata
The Ethereum chain data folder comes from Ethereum desktop clients such as Ethereum Wallet / Mist.
Deleting the chain data can free up ~50GB of space, but will require a lengthy resync.
You will not lose your Ether when you delete the chaindata.
This strategy concerns the chain data of the testnet. For mainnet, see ethereum-chaindata