Set up EventLogging to see how often blocked users are blocked from performing an action via cookie.
This will help us to make sure that this technique works, and actually keeps blocked people out. It'll also be a signal that tells us if we're blocking too many people. The expected range is somewhere between "some" and "holy cow, that's crazy". This is an imperfect metric, but we live in an imperfect world.
See parent task (T5233) for details.