Your child is hooked on MSN, spending hours a day chatting with his buddies. Blocking it seems like an easy way out, but you've been a teenager too and you know this is not the answer.
Molowa has the solution: time control on Mac OS X with Kaohi.
Mac OS X (10.4.6 and later, 10.5.x)
PowerPC or Intel Macintosh 450 MHz or higher
Minimum 256 MB of available RAM
MSN Messenger (or any other msn client)