It just got tougher to post an ad for "adult services" on Backpage.com.

Visa (V) and MasterCard said this week that they will no longer let cardholders make any purchase on the classified advertising website, which is best known for its mature content.

The move was prompted by a campaign waged by Sheriff Tom Dart, of Cook County, Illinois. Dart sent letters to both companies alleging that the classified ad website is used for sex trafficking and prostitution. He estimates Backpage.com generates over $100 million in revenue annually from adult advertising.

"It has become increasingly indefensible for any corporation to willfully play a central role in an industry that reaps its cash from the victimization of women and girls across the world," he wrote.

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Dart said his department has made over 800 arrests related to the site, including more than 50 sex-trafficking and prostitution busts and a juvenile sex trafficking arrest last month.

Dart also sued Craigslist in 2009 for advertising similar services, and that website later shut down its adult section. Dart told CNNMoney that he wants to show the public how "horrific" prostitution and sex-trafficking are.

"We cannot turn a blind on this and pretend this is some twisted 'Pretty Woman' situation," Dart said.

Representatives for the Dallas-based Backpage.com had no comment.

"MasterCard has rules that prohibit our cards from being used for illegal or brand-damaging activities," said spokesman Seth Eisen.

Visa's statement said that the company "has a long history of working with law enforcement to safeguard the integrity of the payment system, and we will continue to do so."

Earlier this year, American Express (AXP) announced that it stopped doing business with Backpage.

According to Dart's office, Bitcoin is now the only currency that can be used on Backpage.com.