linux - how to disable SSH login with password for some users?

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On Linux (Debian Squeeze) I
would like to disable SSH login using password to some users (selected group or all
users except root). But I do not want to disable login using certificate for
them.

edit: thanks
a lot for detailed answer! For some reason this does not work on my
server:

Match User
            !root
PasswordAuthentication
            no

...but can be
easily replaced
by

PasswordAuthentication
            no
Match User root
PasswordAuthentication
            yes

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class="normal">Answer

Try
Match in
sshd_config:

Match
            User user1,user2,user3,user4
 PasswordAuthentication
            no

Or by
group:

Match Group
            users
 PasswordAuthentication
            no

Or, as mentioned in
the comment, by negation:

Match
            User !root

 PasswordAuthentication
            no

Note that match is
effective "until either another
Match line or the end of the file." (the indentation isn't
significant)