I want to restrict spammers from registering as vendors into the site. #

>> To restrict spammers from registering as vendors into the site, all you have to do is check “Approve Vendors Manually” via MultivendorX >> Settings >> General (see the screenshot – https://prnt.sc/0ghgqcEKQaG_ )

Is it possible for a person to be vendor and buyer using a single account? #

>> Both logged in(i.e., an existing customer) and non logged in user can register as vendor via the vendor registration form (see the screenshot – https://prnt.sc/pvr6wx ). So, for logged-in user, we simply make them a vendor along with a customer of the site. For non logged in user, they become a user of the site and a vendor as well.
Hence, vendor(through dashboard page) and customer(through My Account page) can maintain their informations via a single account.

How can I block vendor from accessing the backend ? #

>>To let the vendors stop accessing the WordPress backend, all you have to do is, go to MultivendorX >> Settings >> General >> and disable “ Vendors wp-admin Access” (see the screenshot – https://prnt.sc/rH4fLTq1hxEn ) .

P.s – This option will only be accessible if you are using MultivendorX Pro .