Backlog searching will be disabled by default. Option is available in Settings/Misc

NzbDrone.Web/Models/MiscSettingsModel.cs

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+using System;
+using System.ComponentModel;
+using System.ComponentModel.DataAnnotations;
+using System.Web.Mvc;
+using NzbDrone.Core.Model;
+
+namespace NzbDrone.Web.Models
+{
+    public class MiscSettingsModel
+    {
+        [DisplayName("Enable Backlog Searching")]
+        [Description("Should NzbDrone try tp download missing episodes automatically?")]
+        public bool EnableBacklogSearching { get; set; }
+    }
+}