Your logged in users, as far as Informix is concerned, are shouwn by "onstat -u". Unless they are shared memory connections (ie. telnet) a unix "who" isn't going to show them, nor are they going to see a "wall" command.
To knock them off, its Informix "onmode -z their_sid". Kill -9 isn't going to touch a user, just kill the engine - WHICH YOU DON'T WANT TO DO.