For a list of such filters, see the how-to list. Your site can subscribe to MAPS and block incoming email spam as it happens. These systems all have their own email providers, and should be using them to relay mail. Note that only email connections from these sites should be refused. Other connections are perfectly legitimate.
Many sites have internal newsgroups for discussion of various issues related to the net.
Bring up these subjects, try to get a consensus among your fellow users. Try to get your fellow users to agitate for a relief from spam. The more voices that get involved, the better. Direct them to this web page. 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.