Old ssh sessions left due to /usr/bin/su and do not terminate

Issue

There are old sessions hung when su was used. See tree

init(1)-+
        |-xinetd(7526)-+
                       |-in.telnetd(62134)---login(62137)---ksh(62142)---expect(62233)---su(62250)---ksh(62280)---ssh(64243)2)---ksh(21475)---pstree(48828)

when I just close such window without exiting from su first, it does not kill su and it's childs:

        |-su(62250)---ksh(62280)---ssh(64243)

This way there are already have 100+ old sessions:

ps -ef |grep "/bin/su -"|wc -l
118