Son and Video Game

I have a 14 year old son and he is great. Respectful, polite, doing well at school and fun to be around. As with most kids he’s into computer games and spends a fair amount of time on one in particular. With this game he can interact with his friends but he can also purchase different teams and products. The game requires a card for registration and about 3 years ago my son ran up a bill of over £1,000 buying different teams etc. He was extremely sorry and we took him off the game for about a year. After this we built up trust and let him back on, letting him know it was completely unacceptable and that if he did it again, he would be banned from the game again. He did this another two times (each time about £15-£30) but it was the fact that he had betrayed our trust again. Both times he was sorry and we banned him for months at a time. About 6 months pass, he’s great in all areas and I agree he can go back on the game, few months later and he’s spent about £100. He’s very sorry (again). I just don’t understand how he is so great in all other areas but not in this. He wouldn’t dream of taking money out of my purse and is so honest in all other areas. I’ve took away the game and he won’t be on it ever again. But as far as punishment goes that is it. How do you think I should handle it? Why do you think he does this again and again?