Help: Block IP from Postfix?

Hi All, Somehow a spammer has used my email address to spoof some useless spam sent to a tech mailing list I am on (my address was used in the return path). Of course the mailing list is happily rejecting the spam and sending all the bounces back my way. My poor server is getting loaded (well clam specifically) dealing with the bounces.

In the short term I have shut down postfix, but until I figure out how to block the bounces I'm not happy with bringing Postfix back online and choking my server.

Any ideas? I've looked through the Postfix config in Webmin but can't find anything where I could put in an IP or host to block.

Thanks,

Geoffrey