Deleting and disconnecting a client are two different things.
A client you have invited, and who has established their own account may 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.
Any client, however, may be deleted entirely from the system at any time. When you delete a client, all of their data and history will be eliminated.