Resolution
It can be done with special configuration parameter in <SB_DIR>\Web\web.config file. Where, by default, <SB_DIR> - C:\Program Files\SWsoft\Plesk\Sitebuilder, if Sitebuilder is integrated with Plesk, or C:\Program Files\Sitebuilder if it is standalone installation.
Following parameter should be changed:
<add key="AllowAnonymousSites" value="true"/>
"True" is default value.
After changing the value to “false” guest users will be redirected to Sitebuilder login page, Wizard will be accessible only for Sitebuilder users.