How to disable customers to create websites with certain domain name?
To disallow control panel customers/resellers to create certain domains it is necessary to:
Enable option " Do not let users create DNS subzones in other users' DNS superzones " in Tools & Settings > Server settings (or Server > Server settings on earlier versions of Plesk).
Under administrator account, create subscription for reserved domain name (for example example.com) and create other reserved domains (e.g. exmple2.com and example3.com) in the same subscription.
After that, attempt to create domain with the same name by another customer will produce error: " This domain name already exists " , while attempt to create subdomain for reserved domain (e.g. subdomain.domain2.test) will result in error "Failed domain creation: Unable to create the DNS zone because its superzone belongs to another user".