Random user registers I didn’t authorize

Hello,

All started 4 weeks ago when a user registered to my blog and started to publish content without my permission. Then it happened again and again, 10 users more or less till today. As soon as I see the email from WordPress that says there is a new user, I go and delete the user.
No one knows my password so I don’t understand why this happens. Two days ago I saw on the right sidebar on the META part the Admin Site link which I didn’t put there so I didn’t know it existed. I just had the META widget on the blog but there wasn’t an Admin Site link, that’s new and I didn’t put it there. Then I went to widgets and deleted the META widget. I thought all was solved but yesterday, a new user registered. I don’t know what else to do. I will put the META widget again on the blog, in case you can see something else.

Thank you.

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