You can do nothing about this apart from either deleting them or making a new email address. Email must be the least secure way of sending a message, as anyone can pretend to be anyone else.
Basically, the spammer might have a list of 100,000 email addresses and so he blasts them all with his advert. If these addresses don't exist, he doesn't want to know, so he chooses a random one from the list, in this case yours, and makes that the "from" address. Now any undeliverables get sent back to you.
I once had a spammer use my email address and receieved 5,000+ of these messages a day for about a week. It eventually stopped when they picked some other victim to use.