Thanks to everyone for their thoughts and information.
Normally I get 100-150 spams a day in my held email folder. It's shocking to get to the bottom of the first page and see "5,216 remaining" staring you in the face, and when you report them and get to the bottom of the next page and it says "5,384 remaining" you realize it's mailbombing or newbie spammer with a broken software configuration. It does take a while deleting or reporting using the web interface, about 2.5 minutes for each page of 100. Switching to webmail goes much faster.
I DID call the ISP of the last attack, during the attack. After three transfers and about two minutes on hold listening to distorted classical music (Bach, I believe..) I got their 'Security desk.' I explained my plight, told him the IP address and he said "That customer account will be suspended within the hour. Thank you." and he hung up. What else can you ask for?
I do report all of them so the IP add'y statistics get pushed up, and thankfully SC aggregates and doesn't mailbomb some poor admin trying to keep up.