My ex-wife was a child minder; and she had terms and conditions. If she took a child of yours on, it was agreed that if for some reason (for example you took your child away on holiday) that she still would be paid a retaining fee. In essence this meant she was paid for doing nothing, however, on her part she could've just taken on another child, thereby filling the allotted spaces she was allowed to child mind for.
I seem to remember she was quite ruthless about this retaining fee, and if she didn't get it, she would just fill the vacancy with another child, and her customer would have to find a new child minder. It was her business, and she did well out of it.
What's this got to do with window cleaning? Well, terms and conditions are terms and conditions; they're a two-way-street. If your customer tries to break or change your terms and conditions, you're quite within your right to drop 'em.
It's easier for you to find a new customer (if you've been going for a while and already have a good customer base, work just finds you) than it is for an old customer to find a new window cleaner (who may charge more, who may do work of a lesser quality, who may rob them; the customer doesn't know).
I think as long as you're sticking to your terms and conditions, why worry?