Deleting and disconnecting a client are two different things based on how you've set up your client.
Remember, you determine whether you manually add a client to the system and manage all of their data yourself, or you invite them to have access to their own EN client account. Clients that have not established their own account, may be deleted entirely from the system at any time. When you delete a client, all of their data and history will be eliminated.
Clients who have established their own account, however, may only be disconnected from your account. When you disconnect a client, you will no longer see their profile in your account, but they will maintain their individual account, simply with reduced access to features and functions, until they connect with a Pro again.