Sad news. ISPs are blocking access to the server coordinating the (legitimate) DDOS attacks.

Maybe there are alternatives:

There are simple java programs that will visit websites and ask to reload the page every second. If people just run such programs when they don't use the computer, then that will have the same effect. Maybe you don't need a central server that will coordinate such attacks. Every time you receive spam messages, you can just enter the websites in the program.

This approach may be even better than the screensaver, because it is more flexible. Everytime the spammer changes the websites the attacks will change to the new websites. If millions of people participate, spam will be history. And, of course, there is no way to block such attacks.