Refer to Walt and Forest above. Forest is the founder of the wiki and the forum. You will notice that he only says how to stop notifications, not how to delete an account. This is because the discussion content that someone adds to the forum (threads and posts) is the property of the forum and can't be deleted. The reason for this is that it would be too disruptive to the narrative of existing threads to suddenly remove part of the discussion that others might be responding to.
However, you can delete or minimize your public profile information and inactivate notifications and permissions. To just inactivate your account's notifications, click on the link Forest provides. Or, to see everything you can edit, single-click your profile name and go through each of the items on the drop-down menu. Feel free to indicate that you are inactive anywhere it seems appropriate.