The place for ISPs to stop spam or spambots is the origin... and that's not going to happen unless really CREEPY invasions of data stream is undertaken. And if any ISP does that, they lose SAFE HARBOR protections, which doubly guarantees they won't do a thing.
On our end (receiving) there will be either a nuke it all, a nuke all but, a nuke only by UA/IP, or no nuke at all.
Most will have to make that decision based on their incoming as to how they respond. In my little world I don't worry about the 8-10% that slip through (too much) because the other 88-90% are not.