[bane]

A technique I learned years ago once Outlook starting broadcasting availability is the notion of "defensive scheduling". Basically schedule work time the same as you treat any other meeting. If a meeting request comes in and it conflicts with your previously scheduled work time? Decline.

I've used it pretty successfully, but it requires a little bit of discipline to make sure you have scheduled yourself out a few weeks in advance and that you don't start accepting meetings during that time.

(note: some people will figure out what you're doing when you have 6 hour blocks of time every Thursday. Schedule lots of smaller 30minute and 1hour meetings and people trying to invade your calendar won't know the difference)