I think blocking should absolutely apply to forums. If I see that a person has a penchant for marring people's discussions I don't want them in my thread.
The problem is that the thread isn't your thread -- it belongs to the community. Chess.com clearly came out in favour of this view when they removed the ability for the creators of threads to delete them.
[snip]You'd think, but how often do you see a thread locked upon the OPs request.
And you're right, there seems to be a bit of a mixed message around this.
It's just good practice when you stop to think about it.
If I'm going to put thought and effort into meaningful posts only to have them subject to the whims of the OP (like thread deletion, for example), I'm going to stop putting that thought and effort in before too long.
There are already a couple of places here where you can exercise full control over the discussion and the threads fully belong to you: blogs and groups.