DRAFT - for discussion purposes only
To me the word "encourage" is the same as providing ISPs with free tools. I'm sure 99% of ISPs are in total agreement but like the how-to ability to make it happen. I think if they had the tools available they would use them.
Maybe it's a matter of funding it. Suppose we raised a few million bucks and funded an open source project to create tools for ISPs to clean spambots from their networks? Maybe that's the solution?