If the proxy mimics a normal browser, you would need to block all IP addresses that they crawl from. You should be able to determine the range of IP addresses by visiting several sites that display the visitor's IP address, and putting those URLs into the proxy. You can block these IP addresses using the following line (added to the same place as the BrowserMatch directives above):
This example will block every IP address starting with 1.2.3.x.

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