Use this example to disable the rich text email in all forms.

add_filter( 'frm_rich_text_emails', '__return_false' );

add_filter( 'frm_rich_text_emails', 'disable_rich_text_email_for_specific_action', 10, 2 );

function disable_rich_text_email_for_specific_action( $rich_text_emails, $args ) {

	$target_action_id = 2381; //Replace 2381 with your email action ID

	if ( $target_action_id === $args['form_action']->ID ) {

		$rich_text_emails = false;

	}

	return $rich_text_emails;

}

Learn more on how to find the email action ID.

add_filter( 'frm_rich_text_emails', 'disable_rich_text_email_for_specific_form', 10, 2);

function disable_rich_text_email_for_specific_form( $rich_text_emails, $args ) {

	$target_form_id = 338; //Replace 338 with your form ID

	if ( $target_form_id === (int) $args['form']->id ) {

		$rich_text_emails = false;

	}

	return $rich_text_emails;

}