This week Valve's legal rep sent a formal letter to the operators of 23 Counter-Strike: Global Offensive gambling websites threatening them with punitive action if they don't cease and desist commercial use of their Steam accounts within 10 days.
Such a cease and desist, which a Valve representative confirmed to Kotaku as genuine, is a notable move for a company which has long allowed unregulated CS:GO gambling websites to access Steam with impunity.
Last week, after controversies over rigged CS:GO gambling sites and at least one lawsuit, Valve made a public statement that it would begin cracking down on such sites since their use of Steam APIs and accounts for commercial purposes technically violates Steam's terms of service.
This letter, which specifically outlines that "you should immediately cease and desist further use of your Steam accounts for any commercial purpose" under threat of "all available remedies, including...terminating your accounts." seems to bear out Valve's crackdown threat.
At the time of this writing, at least one of the sites listed in the letter has temporarily shut down in order to ensure its operations comply with Valve's command.