tinymce-docs-4x/_includes/configuration/invalid-elements.md

invalid_elements

This options permits the editor to remove specific elements with TinyMCE executes a cleanup. This option should contain a comma separated list of element names to exclude from the content.

Type: String

Example:

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.