After my problems I ended with changing my guestbook. I found one which seems to work nice ! You can at last leave your messages ! I had to change a little bit the code as there was a bug which forced you to enter a mail even if you said it was not necessary to give it in the admin section. Other nice feature concerning spam : even if you give your mail it's not displayed, you can only be contacted via some kind of form.

If anyone is interested it's the xfguestbook module that I use. For the little hack concerning the email problem, contact me if you're interested.