When I first joined AO I was welcomed fully by mostly everyone and got on with mostly everyone. I did however find the place a little cliquey amongst some long standing members, which is fair enough as they have known each other for quite a while etc.
I made it my goal to speak with and get on with everyone, or at least try to anyway, but sadly the same has happened to me turning me into The Clique Meister General,
I think that when you throw yourself into a forum for the first time you are drawn to fellow members who have also just joined and have yet found a clique to join. It also helps if they have the same car as you,
Some of my forum mates include Hammy156, Magicsy, Pud237, Jason156 etc and I tend to flock around them more than I would newer members due to the fact I know them, know that I like them, and know how to communicate with them etc.
However, saying that, it does not mean that I try to ostracize any members of the forum. It just means that I hang around with certain members I've known for a while.
But isn't that life?
I mean, in the past I've felt pretty ostracized by a couple of long standing members right here on AO, but I think you just have to forget about it and remember who your forum friends are.
There are people in life, such as the ones you speak of, who find it 'fun' to attempt to oust certain members of a forum and, as you say, ostracize them in an annoyingly cliquey fashion.
In all honesty though, with that type of person you just gotta say 'Ya know what? Screw you.' and get on with your life and ignore them because at the end of the day they aren't worth it.