Prevent "participant" role being added to blogs.

I am running a WordPress network with the latest version of Buddypress and bbpress. The site is http://theglobalschool.net/science and it is run for high school students.

My problem is that every time a blog is viewed, that viewer is added as a user, which makes things very confusing after a short while. Is there any way I can prevent this from happening? I noticed there is a checkbox in the settings for forums but it doesn’t seem to resolve the problem.

Thanks,
Rob.