How to prevent 'sudo rm -rf /'
You can add a line like

%admin ALL = !/bin/rm -rf /


to your sudoers file to prevent the execution of the command with the specific options.

Or, if you want to exclude several commands, you can work with command aliases

Cmnd_Alias DANGEROUS_CMNDS=/bin/rm -rf /, /bin/rm -fr /
%admin ALL=!DANGEROUS_CMNDS


You can also try to play it safe by using

Cmnd_Alias DANGEROUS_CMNDS=/bin/rm -rf /, /bin/rm -fr /, /bin/rm / *, /bin/rm * /, /bin/rm -rf / *, /bin/rm -rf * /, /bin/rm -fr / *, /bin/rm -fr* /
%admin ALL=!DANGEROUS_CMNDS


but there still might be other ways you can shoot yourself in the foot with rm so be careful anyway.

But keep in mind that all matching is done on the full string of the command, so sudo rm -rf /Volumes would still work (as would cd /; sudo rm -rf .).

PS: By all means, use sudo visudo to edit the sudoers file and NEVER edit it directly

PPS: I obviously didn't test this with rm (only with /bin/echo instead)