To turn off email notifications for watched threads:
Click on "Watched Threads" in the light blue bar near the top of the boards.
At the bottom of that list, click on "show all watched threads"
In the upper right above the list of all watched threads, above the Last Message column with poster/date, click on "Manage Watched Threads"
from that drop-down menu you can select "Disable email notification"
When you select a thread to watch, you can set whether or not you wish to receive email notifications. For future threads, just select "without receiving notifications" the next thread you watch.
HTH!