I did have something like this a while ago where corrupt ARP replies were being flagged and putting stations into the blacklist table. This would especially happen when someone fired up netstumbler on a client station.
We mitigated the problem by turning on proxy arp, so actual ARP traffic to client stations was rare.
(EDIT: But re-reading your post it would appear this is simply a case of bad DHCP client behavior,
as the log does not show bizarre values for the MAC/IPs. Investigate your options for Enforce-DHCP, it might help clear the user table of the old entries, and if not there is the aaa fast-timers thing.)