Can you block someone in Gmail? I have heard that you can't but I am not that familiar with it just yet? If you can do they know that you are blocking them (i.e. do they get a specific error or something that tells them what you are doing)? If so what does the error say?

Please help as I need to do somehting about someone who is scaring me and I don't want them to know what I have done.

You can include their e-mail address in the SPAM or block addresses filter. Also check out Gmail help section >> (http://mail.google.com/mail/help/more.html)