Finally following the helpful recommendation that appears every time I log in
as root, I disabled the root account by setting the shell to /sbin/nologin
(which seemed to be the popular way to disable accounts in /etc/passwd).  Not
long after, I discovered this is not the correct way to disable the root
account, which leads me to my two questions:

 1 How can Irecover the root login-shell short of re-installing (which I could
   do but prefer to avoid)?  Some sort of crafty reboot into single-user mode,
   perhaps?  This is netbsd sparc 1.5.2 generic.

 2 What is the proper way to disable the root account?