Which product are you referring to here? The Orion iP Address Manager module has a web interface and also has the feature in which if an IP was responding to a scan and is no longer responding, it is marked as "Transient" and is left in that state for a user defined set period of days after which if nothing is still responding on it, it will become marked as available again. You can view more about that module here
At the moment, I am just using the IPAM from Solarwinds Engineers Edition.
You are getting the DNS information as you are doing a lookup for the IP as it's scanned and putting that into the db, so can you do a lookup for the name and see if they match, and if they don't flag it?
Also, if you can flag anything which has gone from transient to available and still has valid DNS records?