I got this trouble today with iChat, and it was frustrating. I was using Yahoo account and while typing, the message is sent automatically without pressing Enter. I was going to go say goodbye with iChat but luckily, this trick seems to solve the problem, hope this help others, too
1. Show your Library folder with this help in my previous post
2. Open Finder, go to Users –> <Your Username> –> Library –> Preferences, find the file com.apple.iChat.plist and delete it.
3. Go go Users-><Your Username> –> Library –> Caches, find com.apple.iChat and com.apple.iChat.cache, if they are there, delete both.
4. Restart iChat to see if problem is solved.