The crontab files have not to be modified by hand, so that the cron daemon can be aware of the changes. This is valid for each user's crontab. If you want to modify/remove the root's cron job, just do
to switch to root, enter your user's password (this assumes you have privileges for system administration) then edit the crontab using
again. If you want completely remove the crontab use option -r, but first take a look (with option -l) if there are other jobs you want to preserve, just in case!