My sentence to these customers is: Do you value a regular window cleaner you can depend on.9/10 said yes. Well like you I also value a customer I can depend on. So I will not be coming again if you are not a reliable customer. Rude, I can be but it`s a two-way street. The day you put your foot down all this crap stops and working becomes a pleasure.
The bit in red: I've had similar conversations but I try to word it another way. Maybe "Well, like you I also value a customer I can depend on. So I can only provide the service to reliable customers. Can I count on you to be reliable?" (or similar). If I want them to cancel, I will use the negatives, as you did. But if I want to keep them, I use the positive version. It all means the same thing, but the brain doesn't process negatives so easily. Listen to a good salesperson's pitch and you'll see what I mean.
Similar thing when I'm presenting a choice to a customer (while speaking). If I'm trying to direct them towards making a particular choice, I have that choice as the last option on the list. Try it, see how it feels, and you'll see what I mean.