When it comes to people I've been on good terms with for a long time, it takes quite a lot for me to decide that I'm done with them. The last such person I decided to remove from my life was a friend that, in the time I had known him, had gone from being delightfully pedantic on occasion to being a full-blown, confrontational wind bag. It got to the point where being in his presence meant biting my tongue 80 percent of the time. It's not like I dismissed him over, as Mendacity puts it, something like littering.
In fact, being that we're Ts, INTJs tend to be quite thick-skinned. I know that, personally, things have been said or done to me that others probably would have been upset by, but to which I took no affront.
When it comes to a room full of random people, I agree completely with Metamorphosis's "streamlining" analogy. The crux of the issue, and this ties into what Proteanmix was saying as well, is that I, and as it would seem from my interactions on this board, other INTJs, would generally prefer to be alone. I'm never actively seeking new friends. At any given time I have a mental list of types of people who would be useful in implementing my current plans and seek those people out. Beyond that, I won't spend time being with people I feel are of no benefit. That's time that could otherwise be spent doing things I enjoy or working on my projects.
However, I am still always, as a rule, cordial to people I meet, and when I have dismissed closer associates, I've done so quietly and without making a huge hoopla of it. I realize that they are people with their own prerogatives and personalities and that our paths have simply either not crossed to any substantial degree, in the case of the "roomful of people", or have parted, in the case of former friends. If people do realize they've been written off, which they usually don't because I'm careful to make it seem as though we simply "drifted apart", they are perfectly within their purview to think, "Well fuck him anyway!"