Storing SSH passphrase in memory but not saving it to the keychain persistently
Set UseKeychain no (the default) and run ssh-add without the -K option.

Note that this does not store the pass phrase in memory; instead, it causes ssh-agent to keep the decrypted key in memory, and subsequent uses of ssh will then work without the need to supply the pass phrase.

If you already have the pass phrase stored in the keychain, you'd need to use the Keychain Access application to find the pass phrase and delete it.