Prepaid Bandwidth disconnections

Hi, I am currently trying to setup a bandwidth prepaid realm, Whereby clients buy 1G, 2G, 10Gigs, or watever. Then I need my radius server to disconnect these people as soon as this number is hit, is there anyway to do this. I am running a cisco PDSN as my NAS, I've already looked at session-timeouts, but my clients would be annoyed being disconnected all the time. Thanks, Michael da silva pereira