I've locked my twitter feed. This has enhanced twitter enormously. It means that I get less annoying tweets from people who are wound up because I didn't happen to share their opinion on something.
At work I see taking personal abuse as being all part of the job. Twitter though is for my own amusement. I often read it at home or on my mobile phone. Having my tweets locked means only people who have chosen to "Follow" me can see what I've written and so consequently only they can reply. It gives the thing a feeling of being a little exclusive club of just under 2,000 people, theoretically all on a similar wavelength.
My colleague Duncan Barkes was the first to do it and to be honest I've only really copied him.
If you'd like to follow me my twitter feed is @NickMargerrison. So far I haven't refused anyone, apart from some woman who wanted to me to go and look at her webcam.