SQL Server Linux Disable Remote Connections
Thanks to David Browne for pointing me in the right direction. Sometimes knowing what terms to search for is the hardest part. I also got direction from this article

So to change the SQL Server to listen on localhost or 127.0.0.1 all you have to do is add the following configuration to the file /var/opt/mssql/mssql.conf:

[network]
ipaddress = 127.0.0.1


then restart the server with:

sudo systemctl restart mssql-server


If you want to get a complete list of all configuration parameters you can run

/opt/mssql/bin/mssql-conf list


Note a sample of the output is like

hadr.hadrenabled                           Allow SQL Server to use availability groups for high availability and disaster recovery
language.lcid                              Locale identifier for SQL Server to use (e.g. 1033 for US - English)
memory.memorylimitmb                       SQL Server memory limit (megabytes)
network.disablesssd                        Disable querying SSSD for AD account information and default to LDAP calls
network.enablekdcfromkrb5conf              Enable looking up KDC information from krb5.conf
network.forceencryption                    Force encryption of incoming client connections
network.forcesecureldap                    Force using LDAPS to contact domain controller
network.ipaddress                          IP address for incoming connections


So you can add any of those to the /var/opt/mssql/mssql.conf file. The syntax is that the prefix before the dot goes in square brackets with the suffix listed below (see above for example).

To test it was working I checked:

1) That the applications using it were up

2) Running sudo nmap -PN &lt;servername&gt; to determine if the port was open

3) Trying to open a connection from a remote computer where I previously could with sqlcmd -S &lt;servername&gt; -U &lt;username&gt; which threw the error

Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : Login timeout expired.
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : TCP Provider: Error code 0x2749.
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : A network-related or instance-specific error has occurred while establishing a connection to SQL Server. Server is not found or not accessible. Check if instance name is correct and if SQL Server is configured to allow remote connections. For more information see SQL Server Books Online..