Unwarranted posts added

Hi,
My Url
http://www.gruvequickfatlossactivitymonitor.com
I succeeded in creating my first site, 7 pages included and
a post for current content to be added (my first post).

No I´ll find additional posts added as unwarranted posts , i.e. not of my own. They are added from outsite, of unauthorized people that not have my consent to jump in to my blog and inflict their posts.
I do delete these permanently but new ones will appear!
My Qquestion:
How to stop these intruding posts (not my category)?
What Settings to use?
Why is the default setting allowing such unwarranted posts?
Please tell me how and why this happens and advise me how to handle it.

Best regards,

Hans Brautigam

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