Valve Software, the maker of the Steam online gaming service, sent cease-and-desist notices to 23 online gambling sites on Tuesday in a move to quell complaints about the service's "marketplace" system.

"Your commercial use of Steam accounts is unlicensed and in violation of the Steam Subscriber Agreement," Valve general counsel Karl Quackenbush wrote in the letter. "You should immediately cease and desist further use of your Steam accounts for any commercial purpose." Quackenbush then advised the recipients that they had 10 days to comply, after which the company will pursue "all available remedies"—including account termination.

The letter began spreading on social media after it was sent to all 23 sites in question, which included CSGO Lotto and CSGO Lounge, and Valve has confirmed the letter's authenticity to Ars. These sites have racked up headlines as of late by gaming Steam's "marketplace" functionality—and turning some popular in-game items, like cosmetic "skins" for characters and weapons, into veritable casino chips. While Steam itself does not contain direct gambling mechanisms, third-party gambling sites have found simple ways to exploit how Steam works so that players can dump skins into a depository, engage in various games, and then collect more skins (which often have material value by way of eBay reselling) or so that players can trade their accumulated skins outright at these sites for direct payouts of money.

Valve was served proposed class-action lawsuit papers two weeks ago, and the suit alleged that Valve essentially aided and abetted third-party gambling sites by not doing enough to disable the sites' middleman abilities when it comes to processing and rapidly trading so many in-game items among thousands of bettors. The company responded indirectly last week by making a public statement condemning third-party gambling sites, but it did not announce firm retaliatory plans at that time.

Valve has yet to offer plans to alter its marketplace functionality, particularly APIs that can be used to process many in-game item transactions in rapid fashion. Beyond that, Valve's only apparent course of action is to continue filing cease-and-desist letters as a retaliatory course of action—which will keep Valve's legal counsel busy if so, as new, free Steam accounts are easy to open in a number of countries.