In component-template-definition, there is the property personalizable.
(See docs)

When set to false, it is expected that the action "Add component variant" is disabled.
(If the selected component derives from a component-template where personalizable is set to false).

The feature did work like expected in p13n-2.0.15.
It does not work anymore with p13n-2.1

Steps to reproduce:

Add the light-module "component-features" to your bundle. (The light-module is attached to this ticket as zip-file).

component-features/
├── decorations
│   └── mtk2
│       └── templates
│           └── pages
│               └── basic.yaml
├── i18n
│   └── module-component-features-messages_en.properties
└── templates
    └── components
        ├── component-restrictions.ftl
        └── component-restrictions.yaml

Create a page with template basic. In main area add component "component restricitions".
Selected the newly created area and check the available actions.