Limiting by referrer is probably the right first step. (And changing the front page text) You want to track usage by the site, not the person, because you can

Limiting by referrer is probably the right first step. (And changing the front page text) You want to track usage by the site, not the person, because you can ask a site to change usage patterns in a way you can't really ask a site's users. Maybe a per IP limit makes sense too, but you wouldn't want them low enough that it would be effective for something like this.