How can I disconnect ssh users, or limit the number of ssh logins?

Setting up maxlogins limit actually works here. Just make sure you use '-' limit type, not 'hard'.

user1          -       maxlogins       1

If you want to kick users who made double login using scponly, here's quick and dirty script, which does that. Put it into crontab, so it executes every minute.

#!/bin/sh

for user in `grep scponly /etc/passwd | gawk -F: '{print $1}'`; do 

    echo "Checking user: $user"
    instances=`ps -u $user| grep scponly | wc -l`
    echo "scponly instances  $instances"
    if [ $instances -gt 1 ] ; then
        echo "Too many connections detected, slaying scponly for user $user"
        if [ -e /tmp/$user ] ; then
            attempts=`cat /tmp/$user`
            echo "Detected $attempts attempts"

            # increment attempts counter
            echo $(($attempts+1)) > /tmp/$user

            if [ $attempts -gt 3 ] ; then
                echo "Blocking $user"
                /usr/sbin/usermod -L $user
            fi

        else
            echo "1" > /tmp/$user
        fi
        killall -u $user scponly

    fi
done

Download script: http://dl.dropbox.com/u/17194482/kill-scponly.sh

The PAM limits won't catch scp or sftp connections because they are not allocated a pty or written to utmp.

You might look at /etc/security/limits.conf for these sort of limits. http://linux.die.net/man/5/limits.conf

There is a 'maxlogins' limit that can be configured on a per user or per usergroup basis. This won't disconnect previous sessions but will restricted concurrent sessions.