Game developer Valve is reporting that it has banned more than 20,000 accounts from its Steam online gaming service after detecting attempts to register illegally obtained copies of its hit PC game "Half Life 2."

A Valve representative posting at the company's user forums site said the accounts were scrapped because users either tried to register the game with a pirated CD activation key or open an account with a stolen credit card.

Valve launched the Steam service two years ago to distribute game updates and other content. With "Half Life 2," Valve used the service to distribute early chunks of game code and to register copies of the game via a product activation process, an increasingly common way for software makers to fight piracy.

In a separate posting, the Valve representative advised against buying Steam accounts on eBay, saying most accounts for sale there were stolen or otherwise illegitimate.