functions.php

// Remove H1 tag from TinyMCE Editor
add_filter('tiny_mce_before_init', 'tiny_mce_remove_unused_formats');

/*
 * Modify TinyMCE editor to remove H1.
*/
function tiny_mce_remove_unused_formats($init) {
    
    // Add block format elements you want to show in dropdown
    $init['block_formats'] = 'Paragraph=p;Heading 2=h2;Heading 3=h3;Heading 4=h4;Heading 5=h5;Heading 6=h6;Address=address;Pre=pre';
    return $init;
}