Neither did I, till Alexjc decided to unilaterally close the gay survey thread. Then I checked and realised I could do the same on any thread I'd started. If you call up a thread that you've started, at the bottom is a button labelled "moderating options" that scrolls down to give you a "close thread" option.
The problem I have with that is what gives anyone the right to decide when a thread should be closed? Basically it means if you start a thread, but don't like any of the answers you get, you can just decide to close all discussion to censor anything you don't like. Maybe I'm a traditionalist, but I believe if you start a topic, then it's out of control where and how it progresses. If it moves off topic, so be it, these discussions are organic things that develop a life of their own. The only reason to close a thread would be if it gets offensive or personally abusive _ and in that case it's up to the mod to do that on the basis of complaints received or his own judgement.
I know the gay thread sailed pretty close to the wind, but nevertheless it was getting a high participation. I may not have liked some of the views expressed in it from james in particular, but I was enjoying debating him. Frankly, I don't think it was Alexjc's call to make.