Non-production domains should have their own API keys, which can be gotten by registering that domain as separate from the production API key. We do not charge for staging sites.
However, sometimes content from staging can bleed into production if a production API key is used on a staging site. This is not uncommon on WP Engine staging sites when a production snapshot is made and the API key is not switched over.
To removed these posts and to make sure they don't show up again in production, you can tell the recommendation system to ignore posts from specific domains.
To do this, simply go to the Control Panel, then go to Domains, and choose the domains you no longer want to show up in production.