If the List of email addresses checkbox is checked under the Settings tab in form edit and email addresses are added in the text field that appears below the checkbox and the form is published, it will not be possible to uncheck the List of email addresses checkbox and to disable the email notifications for this form.

When the form is opened in edit mode and the List of email addresses checkbox is unchecked and the form is published and opened for edit again, the checkbox will be checked and the email addresses of the subscribed users will be listed in the text field.

The same is the behavior if the emails are first deleted from the input field and then the List of email addresses checkbox is unchecked.

As a workaround perform one of the following:

I. Unsubscribe from the bakend UI

If users do not want to receive notifications on form submissions, they can unsubscribe themselves through the backend UI if they have access to the backend. To do this, perform the below steps:

1. Log in to Sitefinity backend with the account of the user who wants to unsubscribe
2. Navigate to Content -> Forms
3. Find the form
4. Click on its Actions link
5. Select Unsubscribe.

II. Unsubscribe all users using the API 

Another approach is to unsubscribe the users using the API. Here is a sample code for a reference for unsubscribing all users who have subscribed for form responses by passing the form name of this particular form:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using Telerik.Sitefinity.Forms.Model;
using Telerik.Sitefinity.Modules.Forms;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Services.Notifications;
  
namespace SitefinityWebApp
{
    public partial class UnsubscribeFormSubscribers : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            // the name of your form under Content -> Forms -> <your form> -> form's Actions link -> Properties -> Name used in code (for developers) field
            string formName = "sf_testform";
  
            var formManager = FormsManager.GetManager();
            var form = formManager.GetForms().Where(f => f.Name == formName).SingleOrDefault();
  
            var subscriptionListId = form.SubscriptionListId;
  
            var notificationService = SystemManager.GetNotificationService();
            var serviceContext = new ServiceContext("ThisApplicationKey", formManager.ModuleName);
  
            List<ISubscriberResponse> formSubscribers = notificationService.GetSubscribers(serviceContext, subscriptionListId, newQueryParameters()).ToList();
  
            foreach (var subscriber in formSubscribers)
            {
                notificationService.Unsubscribe(serviceContext, subscriptionListId, subscriber.Id);
            }          
        }
    }
}

In order to find the form name, go to Content -> Forms and click on the form's Actions link and then select Properties. The form name is displayed in the Name used in code (for developers) field.
 

III. Disable notifications for form responses globally

In order to disable the form notifications globally, perform the following:

1. In the Sitefinity backend navigate to Administration » Settings » Advanced
2. In the treeview on the left, click Forms » Notifications.
3. Uncheck Enabled checkbox.
4. Click Save changes.