Hopefully you will have done a good job for this customer, and they will have at least some feelings of goodwill towards you. While this goodwill may NOT be enough to get them to PAY YOU through the winter, leave on good terms and they MIGHT remember you next time one of their friends / family needs a window cleaner.
Perhaps you know, or perhaps you could make a connection with an up and coming window cleaner who WANTS one off work or is happy to work long days in the summer to service seasonal customers.
If you hand your 'summer only' customers (or less reliable customers) to this person, then you've found a solution for the customer, and you haven't burnt any goodwill, and this means you might get referrals further down the line (maybe from the 'summer only' customer you've helped out, and possibly from this 'up and coming' window cleaner, especially if he is trad only, and there are certain houses he cleans that need wfp.)
Alternatively, perhaps try saying:
The fact is that the service is year round. I'm feeling concerned because I'm needing to cover my expenses through the winter. Would you be willing to accept that I will need to find a customer to replace you now, and I might not have room on the rounds for your home in the spring?
Or, turn the fact that you work year round into a benefit for the customer
eg.
The benefit to you of us working all year round, is that you get a quality, reliable service for a competitive price, AND you'll have nice clean windows if your friends and family visit over Christmas.