The purgeaccounts function was added to FlexHub in the 1403 release last november, it can be used to quickly remove inactive or old accounts. It works very well although I have only tested it with a few hundred accounts but then it doesn't even lag the hub when running. purgeaccounts -h should tell you what you need.
It is a two step operation, first you tell purgeaccounts what profile to work on and the limit in days, it then gives you a list of accounts that would be deleted, and it also gives you a command to run to actually remove those accounts, not until the second command is run anything is actually changed in the hub.
/1d