Obviously, I am not Peter, but I am able to post using his name, user id, email, anything. Is ther any way to prevent "imposters" from hijacking a user's name on a message forum?

No

if an imposter posts a message using your info...would his ip not show up in guestbook admin?

that's why I never even bothered with bravenets's guestbook (or forum) If I were you I would either

-get a paid account on bravenet then upload a guestbook o forum script(that has some authentication) I found then use it

-or find a free hosted guestbook and forum that will do the same things

if you are unable to have a paid account you can try some of the following sites

-http://www.getphpbb.com/ (hosted forum site)

-http://www.tag-board.com/ (hosted 'chat'/'guestbook' script)

there are a lot of others I have not listed (and currently I cannot since I am posting from my ipod)