There is not an option in the Control Panel, you'd have to edit the newreply and newpost script files, the modification would be similar to:
Find:
<input type=\"checkbox\" id=\"dofav\" name=\"dofav\" value=\"1\" checked=\"checked\" class=\"form-checkbox\" />
Replace With:
<input type=\"checkbox\" id=\"dofav\" name=\"dofav\" value=\"1\" class=\"form-checkbox\" />
Please note that this feature will only ever email posts to users who have the "Should anything added to your Follow Lists be emailed to you" preference turned on for their account.