You can exclude anyone from a test as long as you have a way to identify the people you're excluding. If you have some way to identify registered users--perhaps through a cookie--then you can create an Audience that only buckets people who do NOT have that cookie.
Similarly, if the people from your PPC channel are coming through via query parameter, you could create an Audience that only includes people who have a certain parameter.
Here is some more information about how to set up Audiences.
Hope that helps!