Spammers, and other abusers of web services, may be deterred in their attempts to sign up for these services at large scale by making changes to the service registration procedure, where the changes are designed to break the spammer's infrastructure. In one example, a procedure to register for a web service involves presenting a Human Interaction Proof (HIP, or “captcha”) to the user, and gating access to the service upon receipt of a correct solution. If spammers use botnets and/or image capture techniques to initiate registration processes and to transport the HIPs to human or automated solvers, then the registration procedure can be changed in a way that is incompatible with capturing these images, or in a way that is incompatible with receiving HIP solutions from someplace other than the location at which registration was initiated.