Hi,
Disabling the "User registration" from Joomla is not actually disable the user registration within Virtuemart.
Because Virtuemart is an eCommerce website and requires "member" to run. If you want to use the Virtuemart as "Catalog", then there is a parameter in the Virtuemart Configuration, which is called "Use Virtuemart as Catalog", then all the eCommerce components of Virtuemart (including user registration and checkout) will be eliminated.
The practical way of hiding the "Sign In" menu item from the toolbar is this:
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Regards,
Support