The ReflectBlock plugin for MODX Revolution blocks and logs visitors looking for the old reflect snippet, a minor security vulnerability in older versions of MODX.
The best way to block these requests is in your .htaccess file, but this plugin will do it if you don't have access to the .htaccess file or are not comfortable modifying it.
It's also interesting to view the log and see what they are looking for.
Install the plugin with Package Manager. Blocking and logging will begin immediately.