> You might try:
> sudo cat /dev/null > /var/adm/wtmps

   And if you did try that, why wouldn't the user's (pre-sudo) shell
interpret the ">", making the sudo useless?  I practically never use
sudo, so I know nothing, but if I had to guess, I'd try something like
quoting the whole command, and handing the whole thing off to sudo,
possibly adding an explicit shell (as in an earlier suggestion which
looked to me to be much more complicated than should be necessary).  For
example:

      sudo 'cat /dev/null > file_to_be_damaged'
or:
      sudo /bin/sh -c "cat /dev/null > file_to_be_damaged"
or:
      sudo '/bin/sh -c "cat /dev/null > file_to_be_damaged"'

   Of course, I've tried none of these, but they all look more plausible
(to me) than the previous suggestions.