Use the junk email filters in your email software to reduce the spam that you see. Many people say that if you recognize the company then it is safe to unsubscribe, and this may be the case — if you are dealing with a company you have done business with in the past and perhaps you asked to get on their mailing list. In cases like this, if you decide you no longer want emails from them, then it is usually okay to unsubscribe. Keep in mind, though, that many spammers pretend to be other companies to trick you, so be wary.
The best defence? Delete the email if you don't like it, and forget about it. If you don't open it, it cannot hurt you in any way.
There is no way to stop spam completely.