It's better to stay polite and not be a snarky asshole, but if someone fundamentally misunderstands the nature of their professional relationship with you, they need to be corrected. Yeah you might lose business by answering disrespect with disrespect, but if someone is really pushy with trying to manipulate you into doing free work you didn't agree to, it's likely you won't part on great terms anyway. Being honest and straightforward is more important, and good clients won't take it personally being told in clear terms how it is.