Unless you have a mechanism to force the ethernet carrier to go down, or an agent on the host to do so from that side, the only recourse is to use a short lease time as Michael mentioned.
rfc 6704 (previously 3203) are proposed standards that would ameliorate this problem, but adoption has been nonexistent AFAIK.
We use VLANs for security partitioning here as well, since they keep host firewall policies simple and we don't have to worry about DHCP exhaustion that way. We use SNMP and Disconnects depending on the model of the switch.
However, I'm not sure what Brocades can and cannot do. Depending on how flexible your customer is on this matter, another option is to hand out the permanent IP address from the start and instead use port acls installed by RADIUS to restrict the pre-registration machines.