Depends on how much you want to change.
You will need to edit the template files.
You could delete the options to email in the form and leave it as none with nothing else to select. Hide the option completely deleting the column.
Edit the language file to say it has been disabled.
Bla Bla.
But anyway here are the files needed to edit.
Forum/templates
/editdisplay.tpl( for default My Preferences)
/edittopics.tpl (for watched topics)
/editusers.tpl (for watched users)
/editforums.tpl (for watched forums)
Did not have a lot of time to look closer. But they are just small template files when you look at them it will be obvious what options to delete or comment out.
Of course members already getting emails will not be changed that would be a database change to clean them out.
If you need more detail when I have time I will type it in.
BTW,while you are looking at the files there are other email options to consider like the private message email. You could disable that also.
Last edited by Ruben Rocha; 02/22/2010 1:09 PM. Reason: corrected typo error.