And in he pops to screw things up.
If you're using Mail, you can select those emails and bounce them.
<--- popping in to screw things up further...
Chances are the return email address on these emails is an unsuspecting victim of having his machine taken over by malware. So, bouncing is just going make the victim wonder "Where the hell is this coming from?".
When you get an email that has no subject or no body, then check your downloads folder (given you're running Leopard) for a file called No Subject.eml. That's where the executables get saved. Luckily, since you're running Mac OS X you don't have to worry about these things and can simply delete these files.
In the event you received this email using a program such as Outlook running in Windows and if you opened it, then LOOK OUT!!!. Hope you have some type of anti-virus protection in place.