There are no built in contracts through Workify between you and your workers. All communication between a customer and a worker can be done directly on the Workify storefront you create. Beyond this safeguard, you will need to determine a way that best suits you to ensure the workers you bring on board aren't going to "steal" your customers off platform.
The most important thing is to make sure you make it very clear that "poaching customers" and bringing them off-platform is immediately grounds for terminating them as one of your providers. On our storefronts, we try to have the best relationship with our own workers.
With a mutually-beneficial relationship our workers have been very good to us in this realm. You can also make it clear that you can create "custom" services for a provider, should they have a customer they feel they can up-sell -- you'd just create this like any other service, send to the customer as a direct link, and have it remain in "draft mode" on your storefront.