You can filter junk mail in the Apple Mail app, but you should first try to prevent spam email from even getting past your server. If you use a good transportable email address, like Gmail, you'll get most of your junk mail filtered out before it gets to your Mac. Apple Mail can act as a last line of defense if you set up junk mail filtering in the preferences. You can also add your own filters for reoccurring spam email in Gmail and in Apple Mail. If you no longer want to receive mail from a company you have done business with or have subscribed to, be sure to use their Unsubscribe button instead of reporting the email as junk.