If its somebody they see before they are seen, they will walk out of the room. If the client is rude when talking to them they may get up and walk somewhere else and let some other girl see them.
They can refuse to see a client as long as they have a good reason not to see him, but for example - not if they just think he is ugly etc. Must be a legit reason. You come in to this Industry to make the big bucks you take what goes with it without making someone feel rotten about themselves because they are ugly etc.
There is the other side of the coin also, he may be hard in the room or have a huuuuge dick which most girls wont want to take on especially if he is a pounder and refuses to take his size in cosideration.
some clients are considerate, some just really suck and think because they are paying money they can go at it as hard and as long as they want and will keep themselves from coming right up to the last minute.