You don't have to put your status as 'busy' or 'away'. You can simply ignore.
There is no requirement for you to respond if someone says 'hi' to you on chat. Just ignore. For all they know you are fighting off a pack of wild dogs who have broken into your home. Just ignore.
Some people avoid chat at all costs for fear of being interrupted. Some people waste masses of time because they feel that they have to respond to someone who strikes up a chat. You don't need to. Just ignore.
All my friends who work in IT seem to have their chat on 'available' almost all the time. But they are simply happy to ignore.
As far as I'm concerned, the only netiquette issue here is, if someone does ignore you, then you shouldn't get offended and you shouldn't say 'you there?'. After all, for all you know they are fighting off a pack of wild dogs who have broken into their home.
