Rate limit it by ip address (and/or combination of other request/header data), and be transparent about the limits. Have a blurb asking users not to abuse it by

Rate limit it by ip address (and/or combination of other request/header data), and be transparent about the limits. Have a blurb asking users not to abuse it by keeping requests to a minimum. Captcha every nth request. I could go on. Why do you want to limit free uses? Is it because it costs you money, or because you think there's a product worth paying for here?