If you don't want people to talk to you, appear as offline. The way I see it, if you have set the status to busy you are doing something else and might not be able to respond instantly.
At least that is the way I see it - I usually message people instantly that I come up with something I have to say to them, regardless of status - even offline. Then again, if I just want to chat I might not do it if the other person indeed has their status set to busy.
I usually just set it to busy to stop getting the sound, then again the flashing taskbar is just as good as getting your attention as the sound.
In other news, I got a BSoD while writing this post - good old Firefox kept the text tho, so everything was still in the editbox
/ 
