Facebook Inc. and Alphabet Inc.’s Google announced steps to prevent fake-news websites from generating revenue through their ad-selling services, signs that technology companies are moving to address a growing controversy about misinformation on the internet.

A Facebook spokesman said it will explicitly ban sites that traffic in fake news from using the Facebook Audience Network, saying they fall under the category of misleading, illegal or deceptive sites already barred. The audience network places ads on other websites...