It depends on the situation.
If right now I used the chsh
command to choose a shell, then after I login the next time, the chosen shell would be the shell into which I'd be typing commands on the command line.
If you are a system administrator, you have nothing installed that requires tcsh or ksh, and you don't want tcsh or ksh used, ever
, you can just not install them, or if they are already installed, then un-install tcsh and ksh.
If tcsh and ksh are in /bin and you temporarily
don't want them used, no system facility requires tcsh or ksh to run, I suppose you could use this command:
chmod 0 /bin/tcsh /bin/ksh
But if you are going to want those shells to be used sometime later, you'd want to make sure that you know what the correct permissions are, to change them back later.