The next step would be to take a look at one of the regional RIR's (Regional Internet Registry). The RIR's are...
the organizations overseeing the allocation and registration of Internet number resources within a particular region of the world. For the US, the site to review is www.ARIN.net -- the American Registry for Internet Numbers (ARIN). You will need to enter the IP address of the organization's Web server. Look over the results as there should be a sub net mask provided and a range.
While this will provide you the information needed to block your competitors IP address range they could always use a proxy, such as TOR, to get around this restriction.
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