Tag health is determined through multiple detection methods if a supply tag is considered to have high IVT. Flagged tags will show up in the daily Traffic Quality Report emails, along with in the Health column of the supply tags page (seen below). In the UI tag health is updated every hour.
Pre-Bid IVT Blocked requests are included in billable requests. Thus even though these requests are blocked before becoming an impression, you will still want to lower your Pre-Bid IVT Blocked requests to minimize your requests fee. To lower your Pre-Bid IVT Blocked rate, you can run a domain reports for the same time parameters, filtered by the offending supply tag. Here you should take the domains with highest Pre-Bid IVT Blocked rate to inform your partners of offending domains and block them via blacklists on the supply targeting level.