Restricting Block Editing to Registered User

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I'm rather a new boy to Concrete5 and I don't know if the above subject describes my "problem", but...

When a registered user signs-in, navigates to the page assigned for him/her to edit and clicks on the "Edit" button, then two (maybe three) blocks appear to be presented as being "editable". These are "Add to Sitewide Site Name" and "Add to Sitewide Header Nav" plus (possibly) "Add to Header Image" - see attached image file.
If the user attempts to "Add to Sitewide Site Name" (for example), Concrete5 reports "Access Denied" - as it should. This also occurs with "Sitewide Header Nav" but not with "Add to Header Image" - which function might be useful.
So, the user can't do anything with these blocks so ..
why is the function visible?
Is there any way I can hide these apparent functions?

Regards

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