Hello Victor !
About the user to restrict him to one tenant you can do it by just giving him rights on the tenant.
In the “Roles” Tab :
And in “Tenants” Tab, add your custom tenant name, with rights Read&Write if you want the user to be able to save Visualizations, Search, Dashboards. Read&Only for just reading rights.
Don’t forget to create the appropriate “Role mappings”, map the role to the user.
About disabling the Private and Global tenants, I only know how to disable it globally.
In /etc/kibana.yml, you have to add these lines :
Global is a tenant that is shared by all users.
Private is private to each users.
Hope I helped and answered your questions.