You usually can't , but if you mark them as spam, they will go to the junk files. Don't know if you have a blocking system or Spam-Guard , but just marking the ones you don't want as spam will send them in to junk.
What you can do is configure the spam filter(s) on your email server or with your ISP. Mail servers employ applications that filter spam, viruses, etc. (most often SpamAssassin) and they can be adjusted to intercept emails suspected of being spam. They usually have a feature that allows you to create black and white lists that will block or allow specific email addresses and/or IP addresses as well.