FreeBSD has an interesting approach to system initialization: in "/etc/defaults/rc.conf", you will find the standard system startup options. However, you are not expected to make changes to that file (I mean, you can if you really want to... this is Unix ;-)).
Instead, to customize your startup options, the init system allows you to edit the file at "/etc/rc.conf", which only contains overrides to the startup options in "/etc/defaults/rc.conf". The point of this is, no matter how much you mess up your system startup options, even if you completely break the syntax of that file, the main startup options are still available.
So, spend some time browsing "/etc/defaults/rc.conf", then make your overrides in "/etc/rc.conf" in the style of:
firewall_enable="YES"
gateway_enable="YES"
moused_enable="NO"
Now, there is no reason to have an enabling option if it is already enabled in the default config file, thus sendmail_enable="YES" is pointless, but sendmail_enable="NO" will override the default, thus stopping sendmail. (And vice versa for options that are non-enabled in the default conf, but you want to override to enable).