Hello, and welcome to the Discussions.
Never bounce a Junk email -- you are only confirming your existence.
I have been experimenting with some fraudulent messages, alleging to be from some "postmaster", that another poster furnished me, that could not be deleted.
Try creating a New Mailbox, and then Move these messages to that mailbox, and then delete the entire mailbox -- if necessary, delete in the finder. For the situation I have investigated, see:
http://discussions.apple.com/message.jspa?messageID=2835057#2835057
Please report whether there is any actual address in the Return Path Header?
Ernie