This problem is caused by spammers who are using a forged email address pretending to be yours. Most spammers will not use a real return address because they will get their account closed very quickly.
That's why they will often use either faked addresses or real harvested addresses (almost every large domain will have a 'firstname.lastname@example.org', very few will have 'email@example.com'). They also use lists of valid email addresses just for the purpose of diverting bounced messages. If a message bounces, spammers don't want it coming back to their mailbox.
The first thing that you need to do is identify the spammer's mail host and raise a complaint. More information on the matter is available here.
Next, to resolve the problem for your own domain and email addresses, you need to set up an SPF (Sender Policy Framework) record in your DNS zone. SPF records specify the servers, which are allowed to send messages from your domain name. Any messages sent from other servers will be considered fake and thus rejected.