Question about limiting features and licenses

We have a product that contains features (say the A, B and C features).

We are about to put it into production (a web application) for a certain customer.

However, according to my boss, we will deploy the product in the customer's premises. The client will also have the source code.

Now, my boss wants to restrict the features of this client. He wants to restrict the A functionality so that only the B and C features are available to the client.

What are our options? How can we do this technically (restrictive features) and in terms of license?