On a pc
, if you go to user cp and click list subscriptions in the left hand column, you can view all subscribed threads.
There is a radio box at the top of the list that will select all threads on the page.
You then can scroll to the bottom right and there is a drop down menu that will allow you to select "no email notification".
You will have to do this for each page of subscribed threads to remove them all though realistically just doing the first couple of pages will eliminate the problem since anything beyond 2 pages is likely to be a cold or dead thread that is not receiving replies anymore.