Not allow users to change passwd

[Tony Sarendal <tony_(_at_)_torped_(_dot_)_se>]

Hi oBSD'ers.

I have a server (yes my old crappy one) where people can log
in only if they've sent me their public ssh key.

I want to disable their regular Unix password, and make sure
they can't change it.

What is the best way of doing this ?

Thanks

Tony Sarendal