Well not fully back from vacation, but my flight is delayed (it's raining here) and I'm bored.
There's no easy way, but I'll bet that if you look closely, the mail in question is probably actually delivered to addresses at your domain. It's pretty rare to see mail that's actually being sent to username@[mail machine] unless you're posting full headers of emails sent to you somewhere public. Technically, it's possible to block mail to these addresses, but I won't usually do it unless you're actually seeing spam to these addresses.
If you have headers showing otherwise, feel free to pass them along to me via support (I think there's been another thread about this recently).