Google Apps' main benefit is excellent free email service: huge mailboxes, web interface, SMTP/POP/IMAP access, very good spam and virus filtering, and very good reliability. Another advantage is that there are less chances Google mail servers get blacklisted, as often happens with shared servers.
If local mailboxes are not needed, then POP/IMAP access is redundant and there's no need for spam and virus filtering.
However, even if users' email is handled by Google, there are cases where mail is generated by web site scripts, eg. forum notifications, mailing lists, etc. In some cases such scripts can be configured to use remote SMTP server, but in many cases they assume a local MTA is available (eg. PHP's mail() function).
If I understand correctly these are the steps to disable local mailboxes and related services:
For every existing server and sub-server - Uncheck the following: Edit Virtual Server -> Enabled Features -> Mail for domain enabled.
For every domain: Ensure that domain name is not listed in /etc/postfix/virtual - if it does then delete all lines that contain the domain, and finally run postmap /etc/postfix/virtual
Uncheck the following: Virtualmin -> System Settings -> Features and Plugins ->
Mail for domain
New mailbox signup.
Select (check) and click Disable Now and On Boot the following: Webmin -> System -> Bootup and Shutdown ->