Funnily enough I did this the other day.
At the top of the forums you’ll see two bars - the first starts with Forums, What’s New, Members, etc.
The second starts with Forum List, Find Friends, and Watched.
Click on the drop down arrow to the right of Watched and you get Watched threads and Watched forums.
Click on Watched threads. Now look way over to the right, you’ll see a box ‘Manage watched threads’. Again, click on the drop down arrow, then click on the top option, which is ‘Disable email notifications’.
Worked for me, though it took a day or so to work through.