Seth,
You can certainly add a host reservation in your dhcpd.conf file, just append after everything:
------------------------
host name-goes-here {
hardware ethernet (insert mac address here, without brackets)
fixed address x.y.z.w
}
------------------------
Also, dhcp stores the addresses it gives out in a leases file (/var/lib/dhcpd/dhcpd.leases), you can probably delete it, touch, and restart dhcpd (i am not sure if this will break something on a lmce system), or possibly, make offending address expire. Safest way though is to add a host reservation.
Best regards