If he is working like this, he is not a star developer - great software developers understand that maintainability is extremely important. You will probably pay dearly for this in the long run, I would be very direct with him about how serious this is and let him go if he can't start to adjust. I have seen this plenty of times before and it is a ticking time bomb.
To be perfectly honest, I have seen plenty of developers like this and, unless they are just out of school, they won't change. I say cut your lossless now, its only going to get harder to fire him as he continues to spew out more unmaintainable code :)
