Contact publisher: make the e-mail address field NOT changeable by the user

[shafique]

Suggestion: it would be great to have an option in the "Contact publisher" part of an ad that prevents the user from changing the contact email address. So if I respond to an ad and I've registered using the email address john_doe@hotmail.com, then I should not be able to change this when contacting the publisher of the ad. Does this option already exist? Can someone post code or a simple plugin to implement this?

Thanks!