Thanks again for your quick answers.
- I would prefer a guestbook in which guests can not submit a website:
Do you mean that you have to be logged in to post a guestbook entry? If so an option is available for that in WordPress. Goto Settings -> Discussion -> "Users must be registered and logged in to comment". Due to the usage of the wordpress integrated comments this option is also valid for the guestbook.
This is not what I mean.
When a guest writes in the guestbook, he has to submit his e-mail address. But then he is asked to submit his website URL. That is something I don't want.
- I would prefer an option to only enable flat text. No HTML tags and attributes:
I didn't found a wordpress option for that. I also had a quick look if I can find an existing plugin for that, but I didn't found one so far. Perhaps you can have a more detailed look.
I have not found it either, even after long search. Maybe somebody else knows how to solve this?
I hope that helps and I would be happy if you can give me one more star.
When the "thank you" page is in place, you will even get 5 stars!