This paper presents 'For Human Eyes Only' (FHEO), our Firefox extension that enables one to conveniently post online messages, such as short emails, comments, and tweets in a form that discourages automatic processing of these messages. Similar to CAPTCHA systems, FHEO distorts the text to various extents. We provide a security analysis of its four default distortion profiles as well as a usability analysis that shows how these profiles affect response time and accurate understanding. Our results illustrate the security/usability tradeoffs that arise in the face of adversaries that use current, off-the-shelf optical character recognition technology in order to launch a variety of attacks. Two profiles, in particular, achieve a level of protection that seems to justify their respective usability degradation in many situations. The 'strongest' distortion profile, however, does not seem to provide a large additional security margin against the adversaries we considered.