Under content I follow, at the top right if you click the 'Toggle Edit Options' button rather than a change button next to each topic you follow, instead there is a tick box - tick each one you want to change and then at the bottom of the page is a drop down box that will allow you to unfollow multiple posts at once. If you don't see the 'Toggle Edit Options' button then it may be an admin only option in which case - sorry but there is not much we can do about that. The other suggestion would be to change the way you follow things - in your profile settings you can choose to not follow things automatically and then you can proactively choose which things to follow, rather than having to unfollow the things you don't want to follow.