How can I ensure a general contractor pays me?
To the first point, the amount outstanding may be too large for small claims court in your jurisdiction. If that's the case, you could be looking at a more drawn-out process for resolving this in your local courts. Consider speaking to a collections agent or collections attorney about how to handle it. You'll lose about 1/3 of the money in their contingency fee, but you'll be able to get this off your plate and get back to business.
For the future, give some thought to having a written contract, and to making your non-payment policies clear. You shouldn't have to keep performing cleaning services if you're not getting paid. A local attorney can help you figure out how best to do that.