Just focus on the practical, reality of it.
Say they can't take on any of your current customers as of date of leaving for 6 months. That's as good as you're gonna get. And should be easier to prove, if they all start disappearing for someone significantly cheaper.
In reality, if you're big enough to employ, when they leave, they're not instantly going to be able to compete with you. They just won't have the capital, reputation, brand and connections like you have. And that's what a business is really worth, it's not it's ability to clean windows.