Is it possible to avoid adding or inviting my client to their Zoho Books organization while creating a new client from Zoho Practice?

Yes, it is possible to add a new client in Zoho Practice without automatically inviting them to their Zoho Books organization. Check the option Don’t add this client as a user in the organization that will be created for them while creating a client from Zoho Books so that your client will not be added as an user to their organization.