In your .htaccess file, add this code:
<Files xmlrpc.php>
Order Allow,Deny
Deny from all
</Files>
or
<Files xmlrpc.php> order deny,allow deny from all allow from xxx.xxx.xxx.xxx </Files>
Replacing the xxx.xxx.xxx.xxx with the IP of the service you wish to have access your xmlrpc.php file.
or
Search for "Disable XML-RPC" and install the Disable XML-RPC plugin.
Make sure you make a copy of the old file before you do so, just in case you run into any issues.