[DHCP-SERVER] Delete all dynamic leases
http://forum.mikrotik.com/viewtopic.php?f=2&t=70106
if you have only one DHCP-Server you can do the following: /ip dhcp-server lease remove [find dynamic]
if you have several servers, you have to add additional filter to filter by server. /ip dhcp-server lease remove [find dynamic server=dhcp1]