I usually flatline (i.e. quit responding to IMs) in a conversation if one or more of the following happen -- some charges more serious than others:
* user has no profile or incomplete profile with no information
* user is under the age of 18
* user cannot or will not explain how he got my YIM name
* user immediately asks for a cam request without any primer
* user does not respond in a timely fashion
* user gets caught lying about their identity
* user engages in harrassment (e.g. repeated unsolicitied IMs, nonsensical messages, aggressively toned messages or insults, defamatory speech, spam, constant unmet requests for x-rated photos or materials)
In some situations, like the age thing, I politely explain that I'm not comfortable chatting with anyone under the age of legal maturity -- no complaints, usually. Or if you're caught in a lie, I just frown and back away out of the conversation.
As for the rest, sometimes it's just easier to quit responding and let them eventually tire of trying. Sure, it's not direct or explicit, but people can only talk to themselves for so long. I keep my communication open because I like hearing from people, especially people that have caught my attention on the board, made really good points in threads, or that we have some kind of similar background somehow with which to relate to each other. And I don't mind talking to anybody until you give me a reason to do otherwise, y'know.
I think being in a group like this, you know that people have cams -- and since I enjoy it a fair amount, I try to be a little more sensitive about how I'm coming across (demanding or fair) so that I can ask for that same respect out of people.