spam in guestbook

Hello,
I keep on getting spam on my guestbook and keep on spending my time deleting it. You name it, it's all there.. comes in several times per day.
Anyone knows how to avoid that?
Thank you!!
Myriam
http://www.koen-myriam.com

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Thank you. I've made the changes to the 3 files (addentry, form, add.class) but there is something wrong.. I think with the form.php and I can't figure it out.

Can you please look what it did to my screen: http://www.koen-myriam.com/guestbook/addentry.php

What do I need to change to have it look "normal again", I have this black area on the right and no "name" field.

Thanks so much,
Myriam

I just figured it out....

Thanks !!!

Myriam