Root mail huge

In doing some disk cleanup on CentOS, I just found that my /root/Maildir/new directory was 25G.

Does anyone have advise on how to stop this from re-occurring?

Yes, housekeeping!

The user running cron jobs, etc., will get mailed when there is output from the job that would normally go to stdout or stderr. You may well have one or more cron jobs that regularly produces output which will send mails - backups are a prime example.

Whats the best way to defeat this? Send the cron output to /dev/null ?

Not immediately!
What you do is have a trawl through the mails, identify the 'regular offenders' and why they are doing it. If it's 'just' log information of no use then redirect that output to either a fixed file (for each task) so they can be reviewed on a daily basis, or to /dev/null.
If the information is needed, then redirect the output to a file that will either be appened to or contain the run date/time, so that can be reviewed.
Log files will, of course, tale up space so if you redirect all the putput to log files you'll just be moving the problem from one file system/directory to another.
That's why you go through the mails (probaly only really need the last months) to get a decent representation of what is being output and why and then deal with that. Then you keep an eye on the mail root files and perform the above steps for any more output previously missed.
Rinse and repeat. Wax on, wax off ...