I ask myself every so often what is this character doing? Why have I written them (a) the way I have and (b) at all? If I can't answer those questions satisfactorily to myself, then I think it's time for that character to go. I need my characters to have a purpose. And it is fine for purposes to come to an end. For example, I knew Brankaresh would not get away with his treachery and he would be held to account for it. I must admit I enjoyed removing that sexist, treacherous wizard (possibly a tad too much!) but his story had come to a natural end.
So be wary of characters wanting to take you over. Is is their story to tell? Are they adding something positive when they develop this way or are they slowing the plot down, getting in the way somewhat?