Apple has banned a developer from its app store after complaints of fraudulent credit card charges.

The company says that Thuat Nguyen pushed his titles to the app store's best-seller list by hacking into around 400 iTunes accounts. Nguyen was said to use credit card details to boost sales of his comic book apps. At one point, Mr. Nguyen's apps occupied 42 of the top 50 spots in the book apps charts.

Apple says that the developer was banned from the iTunes store "for violating the developer Program License Agreement including fraudulent purchase patterns."

Apple has tightened security in response to the complaints. For iTunes purchases, the company will now ask users to enter credit card security codes more frequently.

Nonetheless, the consumer electronics firm assures consumers that its system remains secure. "The iTunes servers were not compromised. An extremely small percentage of users, 400 of the 150 million iTunes users, were impacted," Apple said in a statement. The company advises people who are worried that their credit card security has been compromised to get in touch with their bank and change their password.