What happens when you reject a friend invite? Does it keep the person from seeing your page? Does it stop them from commenting? Does it delete their comments? I rejected an invite from a person I had already subcribed to and their comments had then disappeared from my page. But the person probably has deleted them because I see you can delete your comments from other people's pages. It's strange, in most blog communities, it's up to the person who owns the page to delete the comments that people submit to their site.
By the way i've just tried it and it works, you can delete comments from other people's pages to answer other help questions about deleting comments.