A system and method to detect and prevent non-human interaction between a client and a web server invokes an effect to change the event generation behavior at the client. Subsequent event streams from the client to the server are analyzed to determine whether the event streams contain events corresponding to expected reactions of a human operator at the client to the effect. Indications of non-human behavior may invoke more direct human testing, for example using a dynamic CAPTCHA application, or may cause a termination of the client/URL interaction.