Depends on your set up. We have a contract with our cleaners and another with our customers. Our cleaners are self employed so we can't have an enforceable clause in their contract saying they can't pinch clients. However we have a clause in our contract with the customers stating that if they approach a cleaner provided by us to work directly for them then they are liable for a one off introduction fee of £750.
Luckily not had to try and enforce it yet but then again we are still a very small company.