You can't. But then why did you assume that I'd be OK with you letting Google to index my replies?

Once you send an email to someone, that content is no longer yours. I understand why you might want it to be. But at its core, email is about sending content to another person, and giving them control of what they can do with that content. Ignoring etiquette, there is nothing stopping them from copying it, forwarding it, printing it, publishing it, etc. Indexing it for their own searches is hardly a bigger step beyond what they can already do with any email provider.

That was a rhetorical question. Sorry if it wasn't clear.

It was just to show that once you start talking about your own comfort and convenience, it wouldn't hurt to consider if you'd be getting these at the expense of other people. This has nothing to do with etiquette, and pretty much everything with basic egoism. "But how would I search" remark above is a good example of this. One's entitlement for comfortably searching their mail history ends where my entitlement for keeping my replies off Google's radar begins. It always works both ways.

Hmm... I suppose explicitly claiming copyright on your email might keep many people from publishing it, particularly if you put a scary warning next to said claim.

Note: I can't think of any reason you can't do this, but whether or not this practice has real legal bite, it could be effective by creating an air of legal uncertainty.

Because if you cared about privacy you would have used Enigmail and then no one cares if the encrypted text is stored on the google server.

And if you don't use pgp you are obviously OK with the recipient email provider reading your mail.