Have a look at Modifying the subnet range of the NAT network in Fusion (2003463) for some relevant information however change "yes" to "no" in the following to turn off the Host Only and NAT DHCP Servers.
answer VNET_1_DHCP yes
answer VNET_8_DHCP yes
To:
answer VNET_1_DHCP no
answer VNET_8_DHCP no