Nice catch Pascal. This is a bug, and we’ll have it fixed in the next release. For now, the way to do it is to edit the crontab from the command line.
Log in to your server, su to root, and run the following command:
crontab -u <user> -e
Where is the user of the crontab file you want to edit. In there, add this line near the top:
MAILTO=""
Note however, that until this bug is fixed, if you edit your crontab via Node/SiteWorx again, it will overwrite this change and you’ll have to fix it from the command line again.