With your example just try that :
[gwailo:~]% who
gwailo console Oct 31 09:54
gwailo ttyp1 Nov 6 10:41
frank ttyp2 Nov 5 01:05 <------- franck uses ttyp2 !!
Now do :
[gwailo:~]% ps -xu
USER PID %CPU %MEM VSZ RSS TT STAT STARTED TIME COMMAND
gwailo 409 3.4 2.5 101308 6452 p1 S 5:20PM 0:01.17 -tcsh
gwailo 187 2.3 7.4 140092 19424 ?? Ss 5:16PM 0:15.39 /System/Lib
frank 513 2.3 7.4 140092 19424 p2 Ss 5:16PM 0:15.39 -tcsh
(...)
look for the PID of frank's session (search for the TT value where you get p2, or find 'frank' as user value),
and then try to do :
[gwailo:~]% kill -9 513
I tryed it on my computer and it worked. But well ... it's maybe so hard (it's a kill -9 !!) for a think like that.
This could help, but maybe you will need a softer method.
Have fun,
pierre