Email alerts are managed on multiple levels.
Conversation AlertsIf you want to disable alerts for new conversations, go to your preferences and disable them.
Watched Thread AlertsThread alerts are managed per thread. If you want to see what threads you have watched, go here.
If you just want to disable all email alerts for all watched threads, click here and confirm.
Email alerts for new threads/posts in specific forums are also managed on at an individual level. To manage these, go here.
If you just want to disable all email alerts for all watched forums, click here and confirm.