Wee update, seems there is a setting in the server configuration that controls this.
If that is the case, it is something that can only GS staff can have access to (thus, needs a ticket). This apparently will disable or invalidate some other features, like controlling users by registering them.
It would be nice to have this setting in the web admin interface to be changed at will without a ticket
# Specific IP or hostname to bind to.
# If this is left blank (default), murmur will bind to all available addresses.
# Password to join server
# To enable public server registration, the serverpassword must be blank, and
# this must all be filled out.
# The password here is used to create a registry for the server name; subsequent
# updates will need the same password. Don't lose your password.
# The URL is your own website, and only set the registerHostname for static IP