I think what's happening is the error messages are still being sent to the E-mail address associated with the user. If that's right, all you need to do is set the error output to null.
Here's a quote from a user comment in the K-base crontab article:
at the end of a command so that stdout doesn't get emailed to you. If you don't want stderr either, do:
You might consider sending stdout and stderr to logfiles instead of to the bit bucket.