I share your pain. Just last Friday a similar incident happened. The account that was spoofed was a forwarding account from my main website, and thankfully Gmail's anti-spam features filtered the bounced mails into my Spam folder.
A quick check with my web hosting providing indicated that it was indeed a spoof attack, and they gave me the same advice, that there was nothing I could do to stop the spoofing so long as open-relay mail servers were still about.

Thankfully the bot subsided over the weekend, and I haven't seen any bounced mail for the last few days.
For all the wonderful innovations I read about on the Internet, it's a wonder though that they haven't patched this long-standing issue yet.