Banks know exactly what you're buying with your credit and debit cards. Now, they're letting marketers know what you're spending money on and what to sell you, by putting ads on your bank statements. But there's a way you can stop it.

"I was a little bit surprised," said Oren Milgram. In his recent online credit cards statement, Milgram found deals for tax software and discount entertainment mixed in with the rest of his charges.

"When you take your debit or credit card and use it at the point of sale, it looks at where you shop, how often you shop, what you've purchased," said Mark Johnson, President of Loyalty Marketers Association.