It seems there is no way to leave your campaign without joining another. I was wondering the least populated campaigns so I don't get so many notification.
I have made multiple postings because there will be days that I won't get any notification and then I get so many notifications at one time that is errors me out and kicks me from the server. Whenever I tried to contact customer service they told me there was nothing they can do. They directed me towards the forums to find a campaign that has less activity.