How to clear DHCP leases in OS X Server (v4.1.3-14S1127) on Yosemite 10.10.4
I found this but can not test since I do not have your set up.


Stop the dhcp server either through Server Admin or from a command line command of sudo serveradmin stop dhcp.
delete the file /var/db/dhcpd_leases. You will need root access. Terminal command of sudo rm -f /var/db/dhcpd_leases
Restart dhcp via server admin or by terminal command of sudo serveradmin start dhcp


Be very careful when deleting files in terminal!