Disable XML-RPC on WordPress

<?php
add_filter( 'xmlrpc_enabled', '__return_false' );

Alternatively, if you use Apache:

# Block XML-RPC
<Files "xmlrpc.php">
    Order Deny,Allow
    Deny from all
</Files>

Or OpenLiteSpeed:

<IfModule mod_rewrite.c>
    RewriteEngine On
    # No backslash before the 'x', and NC makes it case-insensitive
    RewriteRule ^xmlrpc\.php$ - [F,L,NC]
</IfModule>