I'm getting an email notification every time someone posts in a thread I'm subscribed to, even though I've clicked no email notification on my settings. This started a couple of days ago.

 

You show to have a few (old) threads where you did have Email notification enabled. Email Notification can be set in the UserCP Edit Options or per thread. Note the per thread overrides the UserCP Edit Options setting. So you can have a mixture of both. Making changes in UserCP Edit Options does not cancel out the setting per thread for existing subscriptions.

So look at your complete Subscribed list:
https://www.thirdgen.org/forums/subs...n&folderid=all

You will see you have a few (old) threads that show instant such as the Lear Siegler and Your Convertibles threads that both are old threads you participated in years ago. You can manually change those using the checkbox and then in the Select Threads box at the bottom change to No Email Notification and Go.