Techniques are described for detecting inappropriate activities based on interactions with Web sites and other electronic information services. In some situations, the techniques involve analyzing user interactions with an electronic information service in order to determine whether the user interactions are likely to reflect fraudulent activities by the user. In at least some situations, information about user interactions may be analyzed by applying one or more assessment tests that are each configured to assess one or more aspects of the interactions and to provide indications of whether those interaction aspects reflect inappropriate activities. If an analysis of one or more user interactions determines that a user is suspected of inappropriate activity, various actions may be taken to inhibit the inappropriate activity from continuing or recurring in the future.