This is a long-shot, as I can't think of any way you can stop such scams, but...
Is there anything I can do about scam job postings[1] being posted on various free-ads sites online, claiming to be from the company I work for, and giving an email address at yahoo or gmail or some such? If I can get full email headers then I can get the email accounts closed at the various free providers, but this is just a game of whack-a-mole which will go on forever.
What else can I do (if anything)? This clearly isn't good for the company name, and given the current economic situation there are a lot of people who will reply to a "job" which says they can get easy money. And, of course, being the sysadmin you're put with the task of somehow stopping these fake job postings giving people the wrong idea about your company...
[1] Looks like cheque/check fraud, getting you to pay it in and then transfer the non-existent money to elsewhere. Then it gets reversed and you're massively out of pocket.