I should start by asking why? Are you stopping Zimbra for an extended period of time, therefore there's a lot of cron that you don't want to happen? It would seem to me that once you have Zimbra installed you'll normally keep it up and running.
However, if it's going to be down for an extended period of time and you want to turn off that cron:
You probably ought to double check this with someone that knows more about Linux than I to make sure I'm not saying something monumentally stupid, but this should work:
1) 2) Back up your crontab:
or wherever else you want to store it, just don't forget
crontab -l > /tmp/myzimbracrontab
3) This will delete the whole crontab for the zimbra user. Then when you want to re-activate it you simply:
and it'll be re-imported