Peter,
Sounds like a lot of work...
Are there more things you can do to even prevent this stuff from coming to you?
You are far more in tune to this than I, but what about the Capcha/ReCapcha questions; two or three step verifications; two or three step questions, human verification
I assume you are banning the IP addresses. Heck, I'd just block anything with a Russian or Chinese mail domain and or add other countries where this stuff seems to come from.
I fear one day you'll tire of this...