You reply to the email saying 'sorry, but we can't accept your code because...'. (The shortest way to do this is to just reply saying 'NAK', but I agree with Ru

You reply to the email saying 'sorry, but we can't accept your code because...'. (The shortest way to do this is to just reply saying 'NAK', but I agree with Rusty Russell that nak-mails are generally unfriendly and a bad idea: http://ozlabs.org/~rusty/index.cgi/tech/2007-05-04.html http://ozlabs.org/~rusty/index.cgi/tech/2007-05-04.html)