To stop all mail service for an account, you can remove the DNS MX record for that domain, but you'll have to edit the zone file directly in /var/named/domain.com.db and remove the line with the MX record on it, then remove the domain from /etc/localhosts and add it to /etc/remotehosts. Don't forget to restart bind.
This was posted awhile back on the cpanel forums by one of the mods. If you do a search there, you'll find the original thread.
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