Make sure your own site is not a spam-haven Your own site may be a problem site and you don't even realize it. Visit the list of sites being monitored and look for your own site there. If you find it, look at its page and see if it's been a problem lately. If you find that your own site is a problem site, there are lots of things you can do. First, make sure that your fellow users realize that there's a problem.
Many voices are more effective than just one.
Agitate in your internal newsgroups and to your system administrators. Ask them to adopt and enforce anti-spam policies. If your ISP won't change its policies, consider leaving. Remember that your ISP is quite likely shunned at other locations. Your email and netnews may not be getting through, and you won't even know it.