When the next clean is due, do "half" the job. Plenty of soap on the brush eg undiluted virosol to leave every window thoroughly streaky with soap, Then a polite note through the door explaining that you are very sorry that you have been unable to complete the job as you have run out of water and need payment to buy some more pure water making resin. Request for full payment for any previous cleans plus the current clean before you can finish the job. You won't have done anything bad or unprofessional which could potentially get you into trouble (like egging the windows). At least if they still wont pay then you will have the satisfaction of getting some revenge!