You can overload their servers as they pass on any URL to you. Just make sure their resource usage is significantly more than yours. Eg. serving a gzipped response that contains millions copies of the same URL to your site. Ideally, make it not fit in their memory. Or, simply large enough to take really long to compute.

Put it under a URL only you know, then start DoS-ing it.

Of course that requires you to be able to serve a prepared gzipped resonse, depends on your stack.