People recieve spam because they put there emails in places where they are not supposed to. I do not recieve spam. Since I opened my account with my current ISP about a year ago I have not recieved one single item of spam mail. This is because I understand how to prevent its happening in the first place. I would suggest that, rather than try to block incoming spam, educate people on how not to recieve it in the first place, although with thier addresses on a gajillion lists already, that might be difficult. The only other possibility is that spammers might be trying random addresses in your domain, and only keeping lists of addresses that don't bounce. In that case, disabling the bounce messages might help a lot. But education and awareness is the key here.

Be proactive, not reactive.