If you try and delete a NIOS member that is part of DHCP, you will first need to run the following

• Remove the NIOS Member from the FO association. If needed, delete the FO association. Export “DHCP Ranges”, remove all lines except the ones you want to edit, delete the content of “failover_assocation” and set “Server_association_type” to NONE. Then import CSV as “Override”.
• Remove DHCP server from the network. Export IPV4 networks, edit “dhcp_members” and remove the NIOS member from the list. Then import CSV as “Override”.