I've had absolutely none, and like you, I refuse to give a quote if the houseowner has told me they already have a window cleaner. I wouldn't like people taking customers away from me, so I don't do it to others. Some may say it's business, but I'm not about undercutting others, I have my price and stick to it.
If people say their WC is rubbish, they don't like him etc, then I will quote them and then the choice is theirs, but only then will I quote them.
Chris