Hacked guestbook?

Hi, in my guestbook somone from brazilia or so had write messages in my g-book and deleted some of the messages that was there, is there any chance to avoid this sort of hacking. I has happend three or four times now but i have deleted their messages afterwards but they have come back.
Please Help.

If you are using Advanced guestbook 2.2 read http://proxy2.de/forum/viewtopic.php?t=3650 if not get back to us with a link to the guestbook.

Thanks.