Qmail / SpamAssassin - Allow localhost email only?

I have Qmail with Spam Assassin set up on a server (managed through Plesk control panel).

I have several domains with several email addresses at each domain. One email address for a specific domain has recently started getting a lot of spam. That email address is only used for administrative purposes and doesn't even need to receive mail from external servers, just localhost.

How can I configure the server to block incoming mail that is not localhost (or whitelisted) to that specific address but leave the other addresses alone?

Also, if you are familiar with Plesk, will this cause any issues? Plesk likes to manage things, but I haven't seen this option available in Plesk. There is a blacklist and a whitelist, but I see no option to say 'blacklist all except...' or anything like that.