Even if I wanted to do this, I don't think I could take the performance
hit running an access list that large on my incoming ports would create.

Thus the beauty of a Null0 route. The initial SYN from their spam maker
gets through to your SMTP server, but the initial ACK goes into the hole
rather than back out to their spam maker. It costs you a TCP PCB for a
short while on the SMTP server, but there are never enough packets to make
this expensive. And no spam gets through. Try it, you'll like it.