To anyone who struggles with this problem, here is an illustrated guide to resolve it...
1. go to user CP (step 1)
2. Select "Edit Option" on the left of the screen (Step 2)
3. At the "Message & Notification" section (second chapter), go to "Default Thread Subscription Mode" (the second box inside this chapter), click on the drop down menu and select NO EMAIL NOTIFICATION. (Step 3)
Scroll all the way down and click on "Save changes"
You will no longer receive these notifications.