IPs are added to the Truncate list automatically when all recorded events for IP address indicate the message content was spam, scam, virus, or other malware.
There is no way for manual additions. Also IPs are removed from this list automatically, there is no way for manual removal.
When no further events for the IP are recorded, the confidence figure will eventually drop to a point where the IP is no longer listed. In most cases this happens within a day or two. However, if the event was severe and a large number of "bad" messages were detected then it may take a week or more for an IP to be "forgotten".
Is the information on our site wrong or incomplete? You don't want us to monitor your blacklist? Or do you operate a similar blacklist? We would like to hear from you!