The easiest would be to modify your web server to reject requests to that URL. Therefore it no longer causes annoyances. However, if that URL is still being used, your best shot is to reject on a per-IP basis.
You cannot DDOS any server, a DDOS attack works primary on web servers, and the server it's coming from isn't likely to have a web server that matters, since it just redirects DDOSing would be nearly impossible to accomplish without a huge effort.
What may be easier is getting the website shutdown, if you trace the host provider or ISP you can file a claim and possibly get their connection or hosting turned off.
