I got spam in my guest-book. How to delete the spam?

Dear developers.

I’m proud to own a fine made-by-GRAV website to promote my garden services since years, named:
Some nice entries are in my guestbook.
But recently I received a Viagra promotion spam entry.
I looked around in the backend, and am wondering, why the hell I cannot simply delete an entry, like I’d done in my former phpwcms.

Excuse my English, I’m a German.

Thank you people for help.

@Annemarie, See the docs of plugin Guestbook: Where are the messages stored?

Also according the docs ( Visualize messages) , if you’ve got Data Manager installed, you might be able to delete the guestbook item using Data Manager inside Admin.

Mr. Pamtbaau, you helped me a lot!
Now I feel,
I wish I had an admin like @pamtbaau