And how many get cancelled each year because they are not being paid for, and if they do it's much easier to remove a domain name from the internet. Unless there would be an active system with signatures it's very hard to make sure cancelled PI/PA blocks will disappear from the DFZ.
First of all, if a PI block is not paid for it will disappear from RIPE's DB. I would expect many ISPs to have an internal process for IPv6 Peering customers to regularly check the DB. Also, there are a number of groups which regularly analyze BGP announcements looking for things like bogons and ghost-routes. I would expect one or more of those groups to include unregistered PI blocks in their reporting. A greater number of ISPs will probably track these reports to make sure that they are following best practices. --Michael Dillon