Often, it's really annoying to have stream of continuously coming unwanted messages. By only trashing the message won't discourage the senders in spamming our mail.
There is one way you can use to discourage them: by rejecting the message. Technically, it's called Bounce Back. If you're currently using Mail.app (default mail reader for Mac), you can do the trick.
Some articles taken from our resource base, tightly related to current article, to empower you with more knowledge on tweaking the most out of your Mac.