Disable Email field in Identities

This is with regard to the spoofed mail problem I had posted earlier. I have (after getting my hosting account re enabled) found that if you set your email id to an existing email at your domain, then you can easily spoof your emails as coming from that id. This however is not true for email ids that don't exist on your domain or are outside your domain.

So a simple solution would be to disable the E-mail text field in the Identity section of Roundcube. Can anyone guide me as how to accomplish the same?

I know it would still be possible to send such spoofed mails using an email client but most of my users only use webmail, and so I want this spoofing to be not possible through webmail.