Disable mail for cpanel user account

As a host, I'd like a way to disable mail from being sent from the server account without having to suspend all mail from the entire cPanel account.

For example, let's say we have domain.com and the cpanel user is domainco. This account lives on the server cp1.hostingcompany.com. If the site has wordpress or anything else that has a PHP mailer installed, that mail will come from the account domainco@cp1.hostingcompany.com. Aside from delivery issues, envelope mismatches, etc., there doesn't seem to be any way to restrict this mailbox from sending mail other than disabling mail for the entire account via the WHMAPI1 suspend mail function. As I understand it, this would also stop mail from coming from "regular" email accounts. So if the user had mailboxes set up like jsmith@domain.com, or even set one up to use on the website like wordpress@domain.com, you'd have to also allow mail to come form domainco@cp1.hostingcompany.com in order to let those normal inboxes function.

It seems this should be a simple matter but after scouring forums and feature requests there doesn't seem to be any similar request to this. For both spam protection, deliverability, and resource management this would be extremely helpful to control whether a cPanel user account can or cannot send mail without affecting the SMTP functions of other inboxes in the same account.