Standard annotation providers will have the same spam-prevention challenges as existing comment/annotation providers; the standard is just a way to link annotations to the annoyed material when the former is maintained with a separate party from the latter.
That would be up to the endpoints hosting annotations. They would need to build anti-spam or reputation into their server system. Not part of the specification.
If this is going to be used they will have to figure out how to deal with spam, vandalism, and harassment.
I'm especially worried about harassment, honestly. The fact that your domain can be marked up in a browser with no moderation capabilities is just going to lead to worse gamergate style brigading and harassment.