Unwanted user registrations

I am getting user registrations to my site even though I have deselected the option that allows for them as well as changed passwords for the two admin accounts on the site. My site is a multisite setup if that helps.

These registrations are happening more and more frequently.

Any ideas as to why this is happening and how I can fix it?

Example:

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New user registration on your site xxx:

Username: azhmjasklp

E-mail: azhmjasklp@nsabqj.com