You could just try the sensible non-over reactive method,
Speak to them and let them know that to run your business efficiently and be able to keep prices low, you require a prompt payment system, ie before 7 days, If you have not been paid after 8 days send a simple reminder text " Hiya xxxx Don't forget window payment is due" . If after 14 days a sightlier sterner message, " Window cleaning payment is overdue".
When next clean is due skip them , after that you or they have made a long term decision.
They pay prompt you keep
They don't pay and you can dump.
Always remember, a non-paying customer is not a customer. But a slow paying customer can be trained !!