tinymce-docs/_includes/configuration/invalid_elements.md

invalid_elements

The invalid_elements option instructs the editor to remove specific elements when {{site.productname}} executes a cleanup. This option should contain a comma-separated list of element names to exclude from the content.

Type: String

Example: Using invalid_elements

tinymce.init({
  selector: 'textarea',  // change this value according to your HTML
  invalid_elements : 'strong,em'
});

Caution: This option doesn't accept attributes in the list, only elements.

Also see valid_elements and extended_valid_elements for more configuration options.